Call the first number A and the second number B.
A + 2B = 13
2A + B = 20
Solve these equations. You will get the equation
3B = 6. Therefore, B = 2.
Put this into either of the equations to find that A = 9. You can then check by putting A = 9 and B = 2 into both the equations to see if it is true. In this case, it is. Therefore, the solution is A = 9, B = 2.
A = 9, B = 2.