Question 315536
Jack usually mows his lawn in 3 hours. Marilyn can mow the same yard in 5 hours. How much time would it take for them to mow the lawn together? 

this is a rate of work problem

let x = Jack's 3 hours
let y = Marilyn's 5 hours

time together for 2 people = 1/(1/x + 1/y)
time together for 2 people = 1/(1/3 + 1/5)
time together for 2 people = 1/(5/15 + 3/15) (equivalent fractions)
time together for 2 people = 1/(8/15)
time together for 2 people = 15/8 hours = 1.9 hours rounded to nearest tenth
9/10 of an hour is approximately 54 minutes