Rob spent 25% more time on his research project than he had planned. He spent an extra h hours on the project.Khan Academy

Question

The explanation says that he planned to spend $($h$/$25%$)$(100%) hours on the research project. How did they come up with the expression and i don't understand why the h is divided by the 25%. I do understand how it's manipulated to get to 5h but i just don't get how they came up with it in the first place

Answer 1

Let $H$ be the hours Bob planned to spend. Then $$h=\frac14 H.$$ Now the total amount of time spent is $$T=H+h=4h +h=5h.$$ With percents written $$T=\frac{h}{25\,\%}+h.$$