A student scores 70 and 76 marks in two tests. How many marks must the student score in the third test to obtain grade A if the average score for grade A is 80 or more?​

Part (a): table                          d            r (mph)            t (hrs)to river the          d1                40                  15/60from the river    d2              70                  twhere: d = distance (equal in both trips), r =speed, t = time taken in hours, t = unknown time taken to the river.part (b): equation related to distnacesdistance traveled in both trips is equal. therefore,d1 = d2d1 = 40*(15/60)d2 = 70*tthen,40t*(15/60) = 70*tpart (c): solving the equation in part (b) above to calculate t40*(15/60) = 70t70t = 10t = 10/70 = 1/7 = 1/7*60 = 8.57 minutes = 8 minute + (0.57*60) seconds = 8 minutes and 34 seconds.therefore, return trip took 8 minutes and 34 seconds.