1 - In order to solve this problem we have to solve the following equation
20 = 25t - 5t²
2 - In order to solve that equation we have to options
1 - Use the quadratic equation
2 - Factor the expression
For this probem we will facor the expression. The first thing that we will do is divide everything by 5, so we get
20/5 = ( 25/5 ) t - (5 / 5 ) t²
which would give us
4 = 5t - t²
Now we subtract 4 and we get
t² - 5t +4 = 0
then
t² - 5t +4 = (t - 4 ) (t - 1 ) = 0
Therefore there are two answers
t = 4
t = 1
Notice that the answer is correct because
h = 25(1) - 5*(1)² = 20