Question 219900
This is a rate problem
In general:
(1st worker's rate) + (2nd worker's rate) = (rate working together)
given:
1st worker's rate = {{{3/60 = 1/20}}} lawns/min
2nd worker's rate = {{{5/75 = 1/15}}} lawns/min
{{{1/20 + 1/15 = 7/t}}}
Multiply both sides by {{{60t}}}
{{{3t + 4t = 420}}}
{{{7t = 420}}}
{{{t = 60}}} min
It takes them 1 hr to mow 7 lawns working together