Question 987580
A varies directly as b. Let the constant of variability be K. So 
{{{a = Kb}}}

{{{20 = K*4}}}
{{{5 = K}}}

Our equation is {{{a = 5b}}}

You are asked for a value of b that results in a=b. I am not sure if that means find a when b=20 or find the value where a is equal to b.

The only way a=b  is when b=0
If b = 20, the a=100