How many copies of each type were made

Question

Quick copy recently charged 49 cents per page for color copies and 9 cents per page for black and white copies. If shirlee's bill for 90 copies was $12.90, how many copies of each type were made? 

Answer 1

let x = number of color copies; and let y = number of black and white copies; then, 

 

x + y = 90 -------------------> 9x + 9y = 9(90) = 810

 

0.49x + 0.09y = 12.90 ----> 49x + 9y = 1290

 

subtracting the top equation from the bottom equation ---> 40x = 1290 --- 810 = 480 ---> y = 480/40 ---> y = 12

 

then, from x + y = 90 ---> x = 90 - y ---> x = 90 - 12 -----------------------------------------------------------> x = 78