Topic 13. Numbers and digits

Alice writes all counting numbers from 1 to 99 on the black-board. How many times does the digit "9" appear on the blackboard?

Answer explanation

The digit "9" in ones place appears on ten numbers

9, 19, 29, 39, 49, 59, 69, 79, 89,99.

The digit "9" in tens place appears on ten numbers

90, 91, 92, 93, 94, 95, 96, 97, 98, 99.

Therefore, the digit "9" appears 20 times from 1 to 99.

A book has 100 pages. How many digits are used to number all pages in the book?

Answer explanation

The sum of the digits in a 2-digit number is 12. If the tens digit is 2 less than the ones digit, find the number.

Answer explanation

Listing all 2-digit numbers whose sum of digit is 12, we get seven numbers

39,48, 57, 66, 75, 84, 93.

Among these numbers, only 57 meets requirement that the tens digit is 2 less than the ones digit. So, the finding number is 57.

The tens digit of a two-digit number is twice the units digit. If the digits are reversed, the new number is 36 less than the original number. Find the number

Two of my digits are even, but my ones digit is odd. The sum of my digits is 11. I'm between 256 and 299, and I am not 281. What am I?

The sum of the digits of a 2-digit number is 10. The tens digit is 4 times the ones digit. Find the number.

Ksi counted correctly the number of times the digit '2' appears in the whole number list from 120 to 222. What answer did Ksi get?

The sum of the digits of a three-digit number is 6. The hundreds digit is twice the units digit, and the tens digit equals the sum of the other two.

Find the number.

A book uses 216 digits for printing its page numbers. How many pages has the book?

When you multiply 3 whole number, the result is 120. When you add them, the result is 19. Find the largest among these 3 numbers.