Answer:
Vinh must get 87 in the next test to maintain grade B.
Explanation:
Vinh needs 420 points to have a B average in math.
His scores so far are 88, 89, 79, and 77.
Addition of these scores is =

Therefore, the minimum score he can make on the next test to maintain the grade B is :

Hence, Vinh must get 87 in the next test to maintain grade B.