Notice that both 15-foot and 10-foot come before the noun and there is a hyphen between the words. A hyphen is needed when a unit of measurement acts as an adjective.
It doesn't matter what the unit of measure is, if you're putting a number before it, and the number plus the measure describe the noun, make sure you have your hyphen.
Instead of a mile being 5280 feet and each foot being 12 inches, things could be measured in kilometers, which is 1000 meters and 1 meter is 100 centimeters and so on.
In 1960, the General Conference on Weights and Measures honored Tesla's memory by deciding to use his last name – Tesla – as a unit of measure for the strength of a magnetic field.