So basically, this is an equation.
15x + 25 = 100
First, lets do the inverse operations tactic.
Subtract 25 from both sides, causing the 25 and -25 to cancel each other out. 100 becomes 75.
15x = 75.
Divide both sides by 15.
15/15 = 1, so it really just leaves the x by itself because the 1 has no value here.
75/15 = 5.
So, x = 5.