# math

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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

• math -

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)

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