Question 1: Find the quotient of a division, knowing that if 15 is added to the divisor and 5 is added to the divisor, the quotient and remainder are the same?

Let the divisor, divisor, quotient and remainder be a, b, c, d respectively. We have:
a : b=c (remainder d)
a=c.b+d
(a+15) : (b+5)=c (remainder d)
a+15=c.(b+5)+d
a+15=c.b+c.5+d
Which a=c.b+d should:
a+15=c.b+c.5+d
=c.b+d+15=c.b+c.5+d
15=c.5
c=3