End of the two decades the students are 180

The students at the beginning of the two decades are 120

remember that the increase is 3 times the second decade than the first decade. So the ration is 1:3

x+3x=80

4x=80

x=20

So to recap

first decade increase = 20. we do have 120 students

second decade is 3 times that. So we do have an increase of 60.

Total increase in TWO decades is 180

End value is 180
Starting value is 120

\(\frac{180-120}{120}=50%\)

The answer is C

More on % increase or decrese in the new math handout

https://gre.myprepclub.com/forum/gre-ma ... tml#p81625

regards

Great example of picking numbers as a time saving strategy.

If you went down the route of calculating the compounding growth with such a formula, or used a formula to calculate sequential percentage increases, you would have ended up in tricky territory!

The GRE will tempt you to automatically use formulas and will punish you for using them without thinking about the most efficient way to solve the question. Often the most efficient solution involves just simple calculations and/or logical reasoning!

That’s why knowing the rules/patterns etc. is critical for your success.