let x be the number of days needed to complete with 10 students.

First equation,

x= k/10

k = 10x  ——-(1)

Second equation,

x-3 = k/15 —–(2)

solve for x, sub 1 to 2

x = 9

sub x=9 into both equation,

you will get both k = 90

Now, solving the question, let y be the number of students needed to complete the project 4 days earlier. Sub x=9, k  = 90

x-4 = k/y

y=18

The question ask the number of addition students

Hence ans is 8