This problem is Work-time  based .

suppose mowing the yard takes X work.

Now as Jack can maw the Yard in 5 hours so jack per hour work capacity= X/5;

similarly Marlin per hour work capacity= X/3;

suppose jack and Marlin can do it together in Y hours. thus

Y(X/5 + X/3)=X  (Note: Equating the work in both sides.. as X/5 is per unit work so multiplying it with time would give work done by jack . similarly For Marlin)

now

X*Y(1/5 + 1/3 )=X

dividing both sides by X

Y(1/5 + 1/3)=1

now apply the theory of fraction summation which is given by ( a/b + c/d)=((a*d+b*c)/b*d)

here a=c=1

b=5 d=3

thus

Y*((3 +5)/15)=1

multiply both sides by 15/8

thus Y=15/8

Y=1.875 hours

Thus jack and Marlin can do the job together in 1.875 hours