Answer:
6 miles
Step-by-step explanation:
A mile is 1760 yards; and hour is 3600 (=2x1800) seconds.
So a yard per second is roughly two miles per hour (actually 1800/1760 x2 miles per hour, so the error is about 40 in 1800 or less than 1 in 40, 2.5%) and three yards per second is about 6 miles per hour. Upper bound of the estimate would be 6 + 2.5% = 6+0.15 = 6.15 mph.
