Question

A large concrete mixing machine can mix a certain amount of concede in 12 hours, while a smallet one can mix

the same amount in 20 hours, Mixing simultaneously, how long would it cake both machines to the same
amount of concrete?

Answer 1

[(1/12)(5/5) + (1/20)(3/3)](x hours) = 1

(5/60 + 3/60)x = 1

(8/60)x = 1 we can reduce 8/60 by dividing out a 4

(2/15)x = 1 x both sides by 15/2

x = 15/2

x = 7 1/2 or 7.5 hours