ok so the average is 50
average of x number of things is
(add value of all things)/x=average
4 test scores needed
so divide by 4
3 test scores ar known
average is 50
x= missing score
(43+48+42+x)/4=50
times 4 both sides
133+x=200
minus 133 both sides
x=67
a score of 67 is needed