The box of 10 is a better buy because each bar of soap is worth $1.08 instead of $1.26 that you would pay for the box of 14.
Let, the number of students who applied for school = x
Then, it would be: x * 15% = 1800
x * 0.15 = 1800
x = 1800 / 0.15
x = 12,000
In short, Your Answer would be 12,000 students applied for that
Hope this helps!