Question 1190592
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d = cost of 1 doughnut
c = cost of 1 coffee
costs are in pesos


Setting up the cost for Monday
3d = cost of 3 doughnuts
2c = cost of 2 coffees
3d+2c = total cost of the 5 items overall
3d+2c = 66 pesos


Let's solve for c
3d+2c = 66
2c = 66-3d
c = (66-3d)/2
c = (66/2)-(3d/2)
c = 33 - 1.5d
I'll use this later on.


Setting up the cost for Tuesday
2d = cost of 2 doughnuts
3c = cost of 3 coffees
2d+3c = total cost of these 5 new items
2d+3c = 69 pesos


Apply substitution
2d+3c = 69
2d+3( c ) = 69
2d+3( 33-1.5d ) = 69 ... c replaced with 33-1.5d
2d+99-4.5d = 69
-2.5d = 69-99
-2.5d = -30
d = -30/(-2.5)
d = 12
It costs 12 pesos for 1 doughnut


c = 33 - 1.5d
c = 33 - 1.5(12)
c = 33 - 18
c = 15
It costs 15 pesos for 1 coffee


Check for Monday:
3d+2c = 66
3*12+2*15 = 66
36+30 = 66
66 = 66 that works out


Check for Tuesday:
2d+3c = 69
2*12+3*15 = 69
24+45 = 69
69 = 69 that works out as well


We've confirmed we have the correct unit prices for each food item.


For Wednesday, she bought 1 doughnut and 1 coffee
So the amount paid is d+c = 12+15 = 27 pesos
This excludes any kind of tax or extra surcharges.


Answer: 27 pesos
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