Is "They all had 15 minutes waits" grammatically correct? [duplicate]
When you use a quantity and a unit as an adjective, the unit is singular:
A 200-pound man...
A 280-calorie snack...
When the unit is used as a noun, it's plural (unless the quantity is one, of course):
200 pounds of man crashed down on me...
I enjoyed those 280 calories...