You can put this solution on YOUR website!
Frank can cut his lawn in 4 1/2 hours, his wife can cut the same lawn in 4 hours, while their son tom can cut the lawn in 3 1/2 hours. how long would it take to cut the lawn if all three worked together?
Let t = time required when all three work together (with 3 lawn mowers)
let the completed job = 1; (a mown lawn)
Multiply equation by 252, results
56t + 63t + 72t = 252
191t = 252
, just like you had
t = 1.32 hrs working together