Lawn Time

Austin's rate    1 lawn/3 hours    

Jacob's rate     1 lawn/4 hours

Combined

(1 lawn )/ (1 lawn/3hours + 1 lawn/4hours)  =   1 lawn/ (4 lawns/12 hours + 3 lawns/12 hours)

= (1 lawn)/(7lawns/12 hours) =

1 x  12/7   hours                   'lawn' cancels out

= 12/7 hours = 1 5/7 hours =  1 hour  42.9 minutes

Here's an easy way to solve this kind of problem.....

Austin's rate per hour  = 1/3 of a lawn   Jacob's rate  per hour  = 1/4  of a lawn

Add the fractions and take the reciprocal

1/3  +   1/4  =   7/12    →   12/7   ≈   1.71 hours 

[ Thanks, EP....!!!   ]