A tank contains 50 liters of oil at time t=4 hours. Oil is being pumped into the tank at a rate R(t) , where R(t) is measured in liters per hour, and t is measured in hours. Selected values of R(t) are given in the table above. Using a right Riemann sum with three subintervals and data from the table, what is the approximation of the number of liters of oil that are in the tank at time t=15 hours?