84% of 50 is 42. So if your test was graded out of 50 you would've gotten 42/50. If you multiply 50 by 2 (50x2), you have 100, so then you'd just divide the 84% by 2 so you'd get 42%
If there are 3 sons multiply the fraction by 3 and then you have 12/15 because 12/15 is equal to 4/5, so the remaining is 3 out of 15, so divide that between the 3 sons and boom, 1/15th per son.