In an exam, Amy, Bob, Carl and David scored an average of 75% each. Amy scored 60%. The combined scores of Bob and Carl were 40% more than the score of David. What score did David get?
The total score of all four people: 75% × 4 = 300%
The total of score of Bob, Carl and David: 300% 60% = 240%
Think of the 240% as being the sum of two numbers: Bob and Carls combined score and Davids score. Since Bob and Carls combined score is 40% more than Davids score, subtract 40% from 240% and this will give you double what David scored.
240% 40% = 200%
200% ÷ 2 = 100%
David scored 100%.