Question

At the local diner, you can get 3 cups of coffee and 5 doughnuts for $4.20. Alternatively, you can get 9
cups of coffee and 7 doughnuts for $7.80. How much would it cost to get 1 cup of coffee and 1 doughnut? (please be detailed)

Answer 1

Let x be the cost of 1 cup coffee

& y be the cost of 1 doughnut

So, 3x + 5y = 4.20 --------- (i)

&    9x +7y = 7. 80 ----------- (ii)

So, for solving both the equations, multiply equation (i) by 9, so it becomes

9x + 15y = 12.6

9x + 7y = 7.80

--------------------------------------

(15-7) y = (12.6 - 7.80)

8y = 4.8

  y= 0.6

  Put y=0.6 in eq (i)

3x + 5 * (0.6) = 4.20

3x = 4.20 - 3

3x= 1.2

  x= 0.4

So, Cost of 1 cup of coffee , x= $0.4

Cost of 1 doughnut , y= $ 0.6