Alright, lets get started.
In the question given that glacier receded 4 feet a day for a period of 25 days.
Which shows in 1 day, it recedes = 4 feet
means in 1 day, the height of the glacier gets shorter = 4 feet
So, in 25 days, the height of the glacier gets shorter = 4 * 25
So, in 25 days, the height of the glacier gets shorter feet
It means the glacier will get 100 feet shorter at the end of the period of 25 days. : Answer
Hope it will help :)