Let x be that time for Bobby (the value under the problem's question).


Then the time for Billy alone is  (x-35) minutes.


Now,   Bobby makes    of the job per minute,

while  Billy makes    of the job per minute.


Working together,  they make   +   of the job per minute.

Their combined rate is exactly    of the total job, which gives you an equation


     +  = ,      (1)


and now it is your job to solve it !


The first step in solving is to multiply both sides of the equation (1)  by  76*x*(x-35).


You will get a quadratic equation then, and I wish you to get a full success in solving it !