Fifteen metres of cord cost $11.25 at a constant price per metre. How many metres can be bought for$20.25? Enter the number of metres.

Explanation

The cord costs $11.25 divided by 15, or$0.75 per metre, and $20.25 divided by$0.75 is 27 metres. Adding the $9 price increase to 15 would happen to mix dollars and metres, so it is not a valid proportional calculation.