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I need some math assist....

Valued Contributor

I need some math assist....

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
12/2014 750+
3/2017 780+
5/2017 800+
Message 1 of 10
9 REPLIES
Mega Contributor

Re: I need some math assist....

Right 3.2883 or $3.29

Discover IT $17k / US Bank Ace (VSig) $13.5K / US Bank Cash+ (VSig) $13.5k
Sam's Mastercard $13.5k / Walmart Mastercard $10k / Blispay $7.5k PayPal Ex MC $6.1k
CareCredit 5k / Husq $5k / Cap1 QS $4.5k / Barclay Ring $4.85k / Citi DC (WMC) $7.1k
Gardening Date 7/01/16 / MyFico 08: EQ 801 / TU 777 / EX 771 / 06/08/17
Message 2 of 10
Super Contributor

Re: I need some math assist....

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

 

Message 3 of 10
Senior Contributor

Re: I need some math assist....


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.

Message 4 of 10
Mega Contributor

Re: I need some math assist....

+1

Discover IT $17k / US Bank Ace (VSig) $13.5K / US Bank Cash+ (VSig) $13.5k
Sam's Mastercard $13.5k / Walmart Mastercard $10k / Blispay $7.5k PayPal Ex MC $6.1k
CareCredit 5k / Husq $5k / Cap1 QS $4.5k / Barclay Ring $4.85k / Citi DC (WMC) $7.1k
Gardening Date 7/01/16 / MyFico 08: EQ 801 / TU 777 / EX 771 / 06/08/17
Message 5 of 10
Valued Contributor

Re: I need some math assist....

You all rock - thanks!

5/2012: 560 credit scores across the board
12/2014 750+
3/2017 780+
5/2017 800+
Message 6 of 10
Senior Contributor

Re: I need some math assist....


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

Message 7 of 10
Super Contributor

Re: I need some math assist....


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? Smiley LOL

 

Before you app think...
Have you done your research of the CC?
Does it fit your spending?
Do you have a plan for the bonus w/o going into debt?
Can you afford the AF?
Do you know the cards benefits? Is it worth the HP?
Message 8 of 10
Frequent Contributor

Re: I need some math assist....

@longtimelurker

 

Real math. I like your style. 

Message 9 of 10
Highlighted
Super Contributor

Re: I need some math assist....


Dubious wrote:

@longtimelurker

 

Real math. I like your style. 


Thanks!  But just very basic algebra...

Message 10 of 10