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- Re: I need some math assist....

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08-20-2013 03:39 AM

Seems here in Florida that all of the gas stations want you to use their Card (or cash) or pay $0.06/gal more.

But using my Costco TE gets me 3% CB, which was a savings of $0.10/gal. So if using their card gets the gas from $3.39/gal to $3.33/gal - then using the TE, would get it down to $3.29, right?

Or did I forget to carry a 1?

5/2012: 560 credit scores across the board and $500 secured CC

12/2014: 750+ scores across the board and $80k in revolving credit

Get those baddies removed!!!

12/2014: 750+ scores across the board and $80k in revolving credit

Get those baddies removed!!!

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08-20-2013 03:57 AM

Right 3.2883 or $3.29

Discover IT $10.5k / US Bank Ace (VSig) $10K / US Bank Cash+ (VSig) $10k

Sam's Mastercard $8.5k / Walmart Mastercard $8k / PayPal SmartConnect $5.1k

CareCredit $5k / Husq $5k / Cap1 QS $4.5k / Barclay Ring $3.55k / Citi DC (WMC) $3k

Gardening Date 5/8/15 / MyFico 08: EQ 729 / TU 724 / EX 750 / 7/12/15

Sam's Mastercard $8.5k / Walmart Mastercard $8k / PayPal SmartConnect $5.1k

CareCredit $5k / Husq $5k / Cap1 QS $4.5k / Barclay Ring $3.55k / Citi DC (WMC) $3k

Gardening Date 5/8/15 / MyFico 08: EQ 729 / TU 724 / EX 750 / 7/12/15

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08-20-2013 05:49 AM

In more detail, for when gas prices change, suppose a gallon costs $x

Then with your card, the price is (x+0.06)*0.97= 0.97x + 0.0582 (0.97 = 1-3%)

This is less than x when

0.97x + 0.0582 < x

0.0582 < 0.03x

i,e. when x > $1.94 per gallon, which is probably always.

As a check, if gas was $2 per gallon, you would pay 0.97*2.06 = $1.9982, so just a little bit cheaper.

In fullish glory, price of gas is x, non-cash/our-card surcharge is y, cashback rate is z

(1-z)*(x+y) < x

x+y -z*x - z*y < x

y(1-z) < z*x

i.e. use your card if:

y*(1-z)/z < x

Citi: Forward, AA Select Plat, Double Cash, Dividend. Chase: Amazon, Freedom, CSP. Cap One: 2 x QS. Penfed 2x Plat Rewards. US Bank: Cash Plus. Barclays: Arrival, SM. Amex: BC, BCE, ED. Nasa FCU. Ebates, FIA Fidelity Amex, Discover IT Miles.

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08-20-2013 06:32 AM

Repo-ed wrote:

But using my Costco TE gets me 3% CB, which was a savings of $0.10/gal. So if using their card gets the gas from $3.39/gal to $3.33/gal - then using the TE, would get it down to $3.29, right?

Or did I forget to carry a 1?

No idea -- I use a calculator as my arithmetic just isn't there any more.

$3.39 * 0.97 = $3.2883.

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08-20-2013 06:35 AM

+1

Discover IT $10.5k / US Bank Ace (VSig) $10K / US Bank Cash+ (VSig) $10k

Sam's Mastercard $8.5k / Walmart Mastercard $8k / PayPal SmartConnect $5.1k

CareCredit $5k / Husq $5k / Cap1 QS $4.5k / Barclay Ring $3.55k / Citi DC (WMC) $3k

Gardening Date 5/8/15 / MyFico 08: EQ 729 / TU 724 / EX 750 / 7/12/15

Sam's Mastercard $8.5k / Walmart Mastercard $8k / PayPal SmartConnect $5.1k

CareCredit $5k / Husq $5k / Cap1 QS $4.5k / Barclay Ring $3.55k / Citi DC (WMC) $3k

Gardening Date 5/8/15 / MyFico 08: EQ 729 / TU 724 / EX 750 / 7/12/15

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08-20-2013 05:22 PM

You all rock - thanks!

5/2012: 560 credit scores across the board and $500 secured CC

12/2014: 750+ scores across the board and $80k in revolving credit

Get those baddies removed!!!

12/2014: 750+ scores across the board and $80k in revolving credit

Get those baddies removed!!!

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08-20-2013 05:28 PM

longtimelurker wrote:In more detail, for when gas prices change, suppose a gallon costs $x

Then with your card, the price is (x+0.06)*0.97= 0.97x + 0.0582 (0.97 = 1-3%)

This is less than x when

0.97x + 0.0582 < x

0.0582 < 0.03x

i,e. when x > $1.94 per gallon, which is probably always.

As a check, if gas was $2 per gallon, you would pay 0.97*2.06 = $1.9982, so just a little bit cheaper.

In fullish glory, price of gas is x, non-cash/our-card surcharge is y, cashback rate is z

(1-z)*(x+y) < x

x+y -z*x - z*y < x

y(1-z) < z*x

i.e. use your card if:

y*(1-z)/z < x

That hurt my brain

NFCU Flagship $25,000 I AMEX SPG $23,000 I Chase Sapphire Preferred $17,500 I Discover IT $12,000 I

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08-20-2013 05:40 PM

longtimelurker wrote:In more detail, for when gas prices change, suppose a gallon costs $x

Then with your card, the price is (x+0.06)*0.97= 0.97x + 0.0582 (0.97 = 1-3%)

This is less than x when

0.97x + 0.0582 < x

0.0582 < 0.03x

i,e. when x > $1.94 per gallon, which is probably always.

As a check, if gas was $2 per gallon, you would pay 0.97*2.06 = $1.9982, so just a little bit cheaper.

In fullish glory, price of gas is x, non-cash/our-card surcharge is y, cashback rate is z

(1-z)*(x+y) < x

x+y -z*x - z*y < x

y(1-z) < z*x

i.e. use your card if:

y*(1-z)/z < x

That's the formula Chase uses for approvals right?

+ Chase ink =

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08-20-2013 08:21 PM

@longtimelurker

Real math. I like your style.

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# Re: I need some math assist....

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08-20-2013 08:31 PM

Dubious wrote:@longtimelurker

Real math. I like your style.

Thanks! But just very basic algebra...

Citi: Forward, AA Select Plat, Double Cash, Dividend. Chase: Amazon, Freedom, CSP. Cap One: 2 x QS. Penfed 2x Plat Rewards. US Bank: Cash Plus. Barclays: Arrival, SM. Amex: BC, BCE, ED. Nasa FCU. Ebates, FIA Fidelity Amex, Discover IT Miles.

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