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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 \$10.8k
CareCredit 5k / Husq \$5k / Cap1 QS \$4.5k / Barclay Ring \$4.85k / Citi DC (WMC) \$10k
Gardening Date 7/01/16 / MyFico 08: EQ 801 / TU 777 / EX 771 / 06/08/17
Message 2 of 10
Mega 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

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 \$10.8k
CareCredit 5k / Husq \$5k / Cap1 QS \$4.5k / Barclay Ring \$4.85k / Citi DC (WMC) \$10k
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

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

y*(1-z)/z < x

That's the formula Chase uses for approvals right?

Before you app think...
Have you done your research of the CC?
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
Mega 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