Alright lets break it down:
Carol = 60 inches tall
Mindy = ??
Lets turn words into numbers:
60 inches is 20 inches less than (2x) Mindy's height
60 inches + 20 inches = 80 inches
This helps us visualize it better, if 60 is 20 inches less than two times Mindy's height, then that means 80 is two times Mindy's height, right?
80 ÷ 2 = 40 inches
Mindy's height is 40 inches
If this is still unclear, comment back.
Hope I was of use, best of luck.