Short answer: 100
I assume you want to find x.
We want an equation that says that x is equal to 105cm, minus that 5 percent, because that is his height before the 5% was added. That looks like:
x + 0.05x = 105
(0.05 is 5% in decimal form, which is necessary in equations like this).
x + 0.05x = 1.05x
1.05x = 105.
Isolate x by dividing both sides by 1.05
1.05x / 1.05 = x, 105 / 1.05 = 100
x = 100, so he was 100cm tall before summer
My original answer was 99.75. This is wrong because I used the 5% of 105, which is wrong because in the question it says his original height (which is x) had 5% added to it. So its 5% of x, not 5% of 105. This changes the whole equation.
