Mrs Dylan had a sum of money. If she bought 5 chocolate bars and 8 mochi balls, she would have $18 left. If she bought 3 chocolate bars and 9 mochi balls, she would have $26 left. Given that she had $80, how many chocolate bars could she buy at most?
Cost of 1 chocolate bar = 1 C
Cost of 1 mochi ball = 1 M

5 C + 8 M + 18 = 80
5 C + 8 M = 80 - 18
5 C + 8 M = 62 ---- (1)

3 C + 9 M + 26 = 80
3 C + 9 M = 80 - 26 
3 C + 9 M = 54 ---- (2)

Make M the same.

(1) _{x 9}
45 C + 72 M = 558 ---- (3)

(2) _{x 8}
24 C + 72 M = 432 ---- (4)

(3) - (4) 
45 C - 24 C = 558 - 432
21 C = 126

1 C = 126 ÷ 21 = 6

Number of chocolate bars that Mrs Dylan can buy
= 80 ÷ 6
= 13 r 2

Most number of chocolate bars that Mrs Dylan can buy = 13

Answer(s): 13