math
posted by nicole .
zac had some 2c stamps and some 5c stamps.He needed to put 30c on a postcard to mail it. What combinations of stamps could he use? would you please explain,thanks

Assuming we can pay using either all 2¢ or 5¢ stamps...
let's start with
0 2¢ > 6 5¢
1 2¢ > leaves 28 ¢, can't be done with 5¢'s
..
5 2¢ > leaves 20 ¢, paid with 4 5¢
..
10 2¢ > 2 5¢
15 2¢ > 0 5¢
so in the form (2¢ , 5¢) we have
(0,6) , (5,4), (10,2), and (15,0)