1. A drink and a box of popcorn together cost 90p. Two drinks and a box of popcorn together cost £1.45. What does a box of popcorn cost?

Let d stand for the cost of a drink and p stand for the cost of a box of popcorn.

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

A box of popcorn costs 35p.