If a wheel with a radius of 70 inches spins at a rate of 600 revolutions per minute, find the approximate linear velocity in miles per hour.

V = omega * r

v = 600*70=42,000 inches/min

Now, you need to change inches to miles, and minutes to hours:

42,000 inches/min * 1mile/63360inch * 60 min/hour = 39.72 miles/hr

They say approximate, dunno if they mean 'mentally':

42,000 / 63360 * 60 ~ 42,000 / 60,000*60 ~ 42,000/1,000 ~ 42 miles/hr