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jmc

counting and probability junior

Problem

If all multiples of 3 and all multiples of 4 are removed from the list of whole numbers 1 through 100, then how many whole numbers are left?
Solution
We know that every third whole number starting from one must be removed from the list. Since the greatest multiple of less than is , this gives us a total of such numbers. We then consider the multiples of four. Every fourth whole number starting from one is a multiple of four and since , this gives us such numbers. However, we also have to account for the numbers that are multiples of both and which we counted twice. These are the multiples of (the least common multiple of and ). Since , we know that there are multiples of both and . Thus, we have numbers that we removed from the list. Since there were whole numbers total, this leaves us with whole numbers.
Final answer
50