Work/Rate Problem : Help correct my approach

GRE Quant Problem Solving
1

Question : Thomas completed 4 crude paintings in15 minutes. If his goal is to make 20 crude paintings in an hour, Thomas must work approximately what percent faster in the remaining 45 minutes?

I try to calculate the first rate as 4/15 (paintings/minutes). Since 16 paintings are left, the new rate would be 16/45 (since 20 to make in 1 hour). Now, difference in rates – 16/45 - 4/15 = 4/45 paintings/minute. Now I converted to % by multiplying 4/45*100 = 8.88 %

What’s wrong with this? Please also let me know the correct approach.

2

You didn’t use the “percent increase” concept correctly.

As a reminder:

\text{Percent Increase} = \left(\frac{\text{New Value} - \text{Original Value}}{\text{New Value}}\right) \cdot 100