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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?
08-20-2013 03:57 AM
Right 3.2883 or $3.29
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
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.
08-20-2013 06:35 AM
+1
08-20-2013 05:22 PM
You all rock - thanks!
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
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?
08-20-2013 08:21 PM
@longtimelurker
Real math. I like your style.
08-20-2013 08:31 PM
Dubious wrote:@longtimelurker
Real math. I like your style.
Thanks! But just very basic algebra...
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