$5 stamps = a
$3 stamps = b
$0.50 stamps = c
(1)
(2)
(2) divided by 2:
(3)
Hence,
Substituting this into equation (1) will give:
Which will eliminate c from one equation...I don't think that it can be done for another, which would then have allowed you to solve simultaneously because you only have 2 equations but 3 unknowns.