Here's the equation. You can solve it and get the answer.
x is the number of months passing.
115+3x=145-2x
Solve for x and get your answer
Let x be the number of months that have passed.
115 + 3x= 145 - 2x
5x = 30
x = 6
It takes 6 months for Rupert to weigh the same as Michael.
edit: sniped