Missing a solution: Are A and B always equal if A-B=0
I just came across this problem on an online learning app:
I tried to solve it by subtracting 100 from each side then inferring if something minus something else equals zero then those two things must be equal.
Divide both sides by (5-x) and x must be 3.
But brilliant had a different way of solving it that revealed there are two possible values for x.
Where did I make a mistake in my approach to solving the problem?
Is it incorrect to assume that if a minus b equals 0 then a must equal b?