Question

The cost of a lunch of 3 sandwiches, 7 cups of coffee and 1 donut is $3.15. The cost of a lunch of 4 sandwiches, 10 cups of coffee and 1 donut was $4.20 at the same cafe.
How much will 1 sandwich, 1 cup of coffee and 1 donut cost?


s= sandwich cost
c= coffee
d= donut
3s+7c+d = 3.15
3.15 - 9c +7c + d = 3.15

-2c+d = 0
d = 2c
1s+1c+1d = 1.05 - 3c +1c +2c = 1.05

Answer 1

3s + 7c + 1d = 315
4s + 10c + 1d = 420
subtract them

s + 3c = 105
s = 105-3c
sub back into 1st
3(105-3c) + 7c + d = 315
d = 2c

s+c+d
= 105-3c + c + 2c
=105

cost = $1.05