Just divide 100% by 80% and multiply that answer by 5. I'm pretty sure that's how you do it.
We know, 3 feet = 1 yard.
We have to convert feet to yards to calculate the ratio so that the final answer doesn't have any unit.
Therefore, 21 feet =
= 7 yards
Thus, the ratio

The fraction
is already in its simplest form.
Therefore, the given ratio as a fraction in the simplest form without any units is 