### Brain Teasers

# Four Consecutive Natural Numbers

The product of four consecutive natural numbers is 9???4, and we don't know what the ?s are. Find the largest number of the four.

### Answer

The product of four consecutive natural numbers end with a 4, therefore none of them can end with 5 or 0, and their unit digits can be only 1, 2, 3, 4 or 6, 7, 8, 9.Since 20^4=160000 and 15^4=50625, they can only be 16 through 19, and we check that 16*17*18*19=93024.

Therefore, the largest number of the four is 19.

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