In the following question, Greg calculates the percent increase as 100%. Shouldn’t it be 30 - (-15) / 15 = 300% or am i wrong? Also if I’m wrong can someone please explain why.

Let’s go by your idea of 300% increase now, what is 300% increase of 15, we multiply the value by 4 and the answer is 60. Now, is 60 really is 300% greater of 15?

% Increase = \frac{|\text{Final}-\text{original}|}{\text{original}}

or \frac{|60-15|}{15}=\frac{45}{15}=3 \text{ or } 300\%