Question 786122
The problem is tricky because you may mistakenly add their individual working time to the total of 70 minutes.
Suppose Jen needs X minutes to shelve a full cart by herself, and the number of books on the cart is A, then:
(A/(X-10) + A/X ) * 70 = A
X^2 -150X +700 =0
Solve for X, get X = 145 or X = 5. Well if X is 5 then Pat will have a negative working time. Thus Jen uses 145 minutes to shelve a full cart.