Question

What is solve c: 6/15 = 2/c

Answer 1

$\frac{6}{15}$ = $\frac{2}{c}$   Cross multiply (6 x c and 15 x 2)
6c = 30   Divide both sides by 6
  c = 5

Answer 2

It's a proportion.
6*c=15*2
6c=30
6/6=30/6
c=5