This is a proportion equation. 6 yards in 1 hour is 21 yards to x hours.
x=hours it takes to mow 21 yards
1) Set up your proportion: 6 yards / 1 hour = 21 yards / x hours
2) Now you cross multiply. (6*x)=(1*21)
3) 6x=21
4) Divide both sides by 6
5) x=3.5
So Johnny can mow 21 yards in 3.5 hours.
Hope this helps! :)
