Question

A number has the digits 9 and 7. To the nearest 10, the number rounds to 100. what is the number?

Answer 1

97

Explanation:

The digits formed by 9 and 7 (without repetition) are 79 and 97. In rounding off a number to the nearest tens, the number in the unit position is considered.

If that number is less than 5, it is changed to 0. However, where the number is above 5, 1 is added to the number in the tens place with the number in the unit place changed to 0.

Going by the above explanation, 79 rounds off to 80 while 97 rounds off to 100.

Answer 2

The two combinations of two-digit numbers we could have would be 79 and 97.  The only one of those that would round to 100 is 97.  If we added another digit to the number, it would be over 100.  The very smallest three digit number we could have with these two numbers in it would be 179; that would not round to 100.  That means 97 must be our number.