Answer: He took 3 hours to dig across the garden
Step-by-step explanation: First of all, we calculate the radius of the circular garden. Digging across the garden simply means drawing a line from end to end along the middle of the garden (in other words, the diameter).
The circumference has been given as 21pi. If the circumference is calculated as
Circumference = 2Pi x r, then
21Pi = 2Pi x r
By cross multiplication we now have
(21Pi/2Pi) = r
21/2 = r
10.5 = r
If the radius has been identified as 10.5 yards, then the diameter can be derived as radius times 2
Diameter = 10.5 x 2
Diameter = 21 yards
However, he is digging at a rate of 7 yards per hour, which means by the time he has dug 21 yards he has spent;
7 yds = 1 hr
(7 x 3 = 21 yds) = 1 x 3 hrs
21 yds = 1 x 3 hrs
21 yds = 3 hrs
Therefore he took 3 hours to dig across the garden.