john's work is 1/7 lawn per hour
larry's is 1/5 lawn per hour
so each hour, by working together, they mow 1/7+1/5 lawn
1/7+1/5=5/35+7/35=12/35
12/35 times x hours=all lawn=1
times both sides by 35/12
x hours=35/12
xhours=2 and 11/12 hours
it would take 2 and 11/12 hours or 2hhours and 55 mins