Ravi has some money. If he buys 40 doughnuts, he will have $5 left. If he buys 50 doughnuts,he will need another $15. How much does one doughnut cost?

Difference in doughnuts = 50 - 40 = 10
Difference in money = 15 + 5 = 20

20 ÷ 10 = $2
Each doughnut costs $2

Check:
40 x 2 + 5 = $85
50 x 2 -15 = $85
Ravi has $85 at first.

Each doughnut costs $2