There are two unknowns - the number of tens (Tn) and the number of twenties (Tw). We need two equations to solve. Let T be the total income of $350.
We know that T = 10*Tn + 20*Tw
We also know that Tn = Tw + 2
Use the equation for Tn in the first:
$350 = 10(Tw+2) + 20Tw
$350 = 30Tw + 30
Solve for Tw and you get 11, the number of twenties. Use that to solve the second equation to determine that Tn is 13.
Check the conclusion. Does 11 twenties and 13 tens add to $350?