A traffic engineer monitors the rate at which cars enter the main highway during the afternoon rush hour. From her data she estimates that between 4:30 P. M. and 5:30 P. M. the rate R(t) at which cars enter the highway is given by the formula R(t) = 100(1 − 0.0001t2) cars per minute, where t is the time (in minutes) since 4:30 P. M. Find the average rate, in cars per minute, at which cars enter the highway during the first half hour of rush hour