David and Colin together have 210 marbles. If the number of David's marbles is equal to 40% of Colin's marbles, how many marbles does Colin have?
Set Colin's marbles as 100%, then David's marbles as 40%.
Then, the sum of their marbles is 100% + 40% = 140%
Use the formula: Whole = Part ÷ Percent
Thus, Colin's marbles: 210 ÷ 140% = 150