Write a story for 3/9=21/k

David contributes 3 dollar for every 9 dollars that Ken contributes, how dollars will Ken contribute when David contributed 21 dollars.

Step-by-step explanation:

David and Ken are contributing money to buy a textbook.

They agreed, that every 3 dollars that David contributes, Ken should contribute 9 dollars.

Find how much Ken contributed if David when David contribute 21 dollars.

If we set up a proportion equation to solve this problem, we get;