So, we can start by multiplying the numerator and denominator of 6/15 by 4:
6/15 x 4/4 = 24/60
Now, we have a fraction with a denominator of 60. To rewrite this fraction as a denominator of 20, we need to find an equivalent fraction with a denominator of 20.
To do this, we can simplify the fraction 24/60 by dividing both the numerator and denominator by their greatest common factor, which is 12:
24/60 ÷ 12/12 = 2/5
Therefore, 6/15 is equivalent to 2/5 when the denominator is 20.