Question 984952
Jenny, 1/j
Penny, 1/p
Lenny, 1/L
Jenny & Penny, 1/2
Penny & Lenny,  3/5
Jenny & Penny & Lenny, 1/1=1


The rates of any of them together are the sum of their rates, so you can write three equations for a system of equations.  Numerator 1 is for 1 gallon of ice cream, and the j, p, and L are the time for each to eat the one gallon.