Suppose that a volcano is erupting and readings of the rate r t at which solid materials are spewed into the atmosphere are given in the table The time t is measured in seconds and the units for are tonnes metric tons per second Use 6 equal subintervals t 0 1 2 3 4 6 r t 2 12 18 36 46 56 60 a Give upper and lower estimates for the total quantity Q 6 of erupted materials after 6 seconds Q 6 tonnes lower estimate Q 6 tonnes upper estimate b Use the Midpoint Rule to estimate Q 6 Use 3 equal subintervals