The area of a circular sinkhole increases at a rate of 420 square yards per day. How fast is the radius of the sinkhole growing when its radius is 50 yards?
Area = πr^2
d(area)/dt = 2πr dr/dt
given: d(area)/dt = 420 yds^2/day
find : dr/dt when r - 50 yds
420 = 2π (50)dr/dt
dr/dt = 420/(100π) = 21/(5π) yds/day
or appr 1.34 yds/day