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John has an ample supply of 10-cent stamps and 15-cent stamps.  He will buy an ample supply of stamps of one more denomination, which is a whole number.  He needs to be able to combine the stamps to make any value of 30 cents or more.  In cdents, what is the greatest denonmination of stamp he can buy which will enable him to do this?

 Nov 18, 2019
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John has an ample supply of 10-cent stamps and 15-cent stamps. 

He will buy an ample supply of stamps of one more denomination, which is a whole number. 

He needs to be able to combine the stamps to make any value of 30 cents or more. 

In cents, what is the greatest denonmination of stamp he can buy which will enable him to do this?

 

My attempt:

\(\begin{array}{|rcll|} \hline \text{cent} & \text{any value of } x \text{ cents or more} \\ \hline 2 & x = 14 \\ 3 & x = 18 \\ 4 & x = 22 \\ \color{red}6 & \color{red}x = 30 \\ 7 & x = 34 \\ 8 & x = 38 \\ 9 & x = 42 \\ 11 & x = 90 \\ \hline \end{array} \)

 

laugh

 Nov 18, 2019

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