Calculus · Easy problem

Cars pass a traffic sensor at an increasing rate during the morning. Readings (cars/min) at $t=0,30,60,90$ minutes are $20,28,35,40$. Using a LEFT Riemann sum with these $30$-minute subintervals, estimate the total number of cars passing in the $90$ minutes, and state whether this estimate is too low or too high given that the rate is increasing.