The answer is
I'll show you two ways of doing this, an easy one and an easier one.
The easiest way of solving this problem is by setting up 2 equations
SInce we're dealing with mass ratios, we'll assume that the final solution has a fraction a of the 20% solution and a fraction b of the 60% solution; a + b must be equal to one since we're dealing with fractions of a total.
Now, solving the system of equations will produce
So your solution needs
The second way of doing this is by using the rule of the cross
You start by placing the starting concentrations on the top row - 20% on top-left, 60% on top-right; you place the desired concentration in the middle - 30%.
Then you substract according to the BLACK lines to get the parts of each solution needed to make the desired one. So,
The final solution will have
So, once again, the mass ratio between the two solutions will be