if it takes 3 people 8 hours to wash 15 cars, how long will it take 2 people to wash 7 1/2 cars

Question

math riddle

Answer 1

If 3 people can finish 15 cars in 8 hours, then when we divide by three, one person can finish 5 cars in 8 hours.

Or:

1 person finishes 5/8 per hour

2 people means cleaning 10/8 cars per hour or 5/4 per hour

 

7 ½ = ¹⁵/₂ (converting the cars needed to wash to fraction form)

⁵/₄ x = ¹⁵/₂ (the amount of washing over time will equal ¹⁵/₂ cars washed)

Multiply both sides by ⁴/₅:

x = ¹⁵/₂ * ⁴/₅

15 and 5 can factor out a 5 to cancel out:

x = ³/₂ * ⁴/₁

2 and 4 can factor out a 2 to cancel out:

x = ³/₁ * ²/₁

x = ⁶/₁

x = 6

It will take 6 hours.