If we let x be the price for soft tacos and y be that of double deckers,
then from the question, we'll have
3x + 3y = 11.25
x + y = 11.25/3
x + y = 3.75 (1)
in addtion, we also have
4x + 2y = 10.00
2x + y = 5.00
y = 5.00 - 2x (2)
Substituting y from equation (2) into equation (1),
x + (5.00 - 2x) = 3.75
-x = 3.75 - 5.00
x = 1.25
y = 5.00 - 2(1.25) = 2.50