Question

A drink and a box of popcorn together costs 90p.2 drinks and a box of popcorn together costs 1.45. what does a box of popcorn cost

Answer 1

d + p = 0.9
2d + p = 1.45

d + p = 0.9
d = 0.9 - p

2(0.9 - p) + p = 1.45
1.8 - 2p + p = 1.45
1.8 - p = 1.45
-p = -0.35
p = 0.35

Answer 2

`x-cost\ of\ drink\\y-cost\ of\ popcorn\\\\ \left \{ {{x+y=0.9\ \to\ x=0.9-y} \atop {2x+y=1.45}} \right. \\\\2(0.9-y)+y=1.45\\1.8-2y+y=1.45\\-2y+y=1.45-1.8\\-y=-0.35\\y=0.35\\\\Box\ of\ popcorn\ cost\ \$0.35`