Lets say Y is the amount they will save.
So Melinda's savings could be described by the equation Y=22+10X, where X is the number of weeks. The same goes to George, but his equation would be Y=10+13X.
To save the same amount, their equations must match:
22+10X = 10+13X
Solving this, one gets
12 = 3x
=> 4=x
So the answer is 4 weeks.
