From Quant concepts- need further explain for solution

GRE Quant Problem Solving
1

Screenshot 2021-05-26 at 05.18.28
Screenshot 2021-05-26 at 05.18.281378×361 51.3 KB

I don’t understand the solution to this- would appreciate if someone could solve! Thanks

2

Suppose you had 100L of air to begin with.
In the first stroke, you will remove 50L
In the second stroke, you will remove 25L ( 1/2 of 50L)
In the third stroke, you will remove 12.5L ( 1/2 of 25L)
In the fourth stroke, you will remove 6.25L ( 1/2 of 12.5L)

All in all, you have pumped out (50 + 25+ 12.5 + 6.25)L of air, which is 93.75L
93.75/100 = 15/16