text: assume you have y tests\n\nThen according to the information\ny x 90 = subtotal + 97  ...1(equating total marks)\n\nAlso\n\ny x 87 = subtotal + 73  ...2\n\nSubtracting equation 2 from 1 you get\n\n3y = 24\nhence y= 8 tests\n\nIn the equations y x 90 is number of tests x average = total\nsubtotal is the total without the marks of the last test