Number of ways the second grade students can sit on the first row is 5! = 5P5
Number of ways 15 first grade students can sit in the remaining 20 seats is 20P15
Therefore, required number of ways is 5P5 * 20P15. (option B)
Let

where we assume |r| < 1. Multiplying on both sides by r gives

and subtracting this from
gives

As n → ∞, the exponential term will converge to 0, and the partial sums
will converge to

Now, we're given


We must have |r| < 1 since both sums converge, so


Solving for r by substitution, we have


Recalling the difference of squares identity, we have

We've already confirmed r ≠ 1, so we can simplify this to

It follows that

and so the sum we want is

which doesn't appear to be either of the given answer choices. Are you sure there isn't a typo somewhere?
I think you should multiply 5x4=20.
It looks like the shadow for the lamp only went up 8 feet so I assuming it would go up 8 feet as well for the guy
Total students = 13 + 9 + 3 = 25
ballgame = 9/25 = 0.36 = 36%
answer
B. 36%