In your mind, you could think 100% more is 200% and 50% of that is another 100% so 200% increase. Let's see on paper. x will be her current hair length.
x + x + .5(x + x)
x + x is simply her hair length doubling. This is when it grew 100% longer. Then, from that, we have 50% more, so we have to add 50% of the x + x.
2x + .5x + .5x
3x
Subtract our starting length.
3x - x = 2x
The difference is 2 hair lengths, or 200% her original length of hair.