To create a one-variable, one-step equation based on the information provided:
Let \( q \) represent the number of quarters. Since Matthew has twice as many quarters as dimes, and he has 20 dimes, we can express the relationship as:
\[ q = 2 \times (\text{number of dimes}) \]
Substituting in the number of dimes:
\[ q = 2 \times 20 \]
So, the equation that represents this situation will be:
\[ q = 40 \]
The final filled-in equation is:
40 = 20 (with "q" representing the quantity of quarters). However, for the boxed answer specifically requested of the format ___q = 20, we use:
2q = 40