1.  I am thinking of a two-digit number. The tens digit is twice the ones digit. The ones digit is 4. What number am I?

       I will not solve it for you / instead of you.  I sincerely believe that every person
       who reads it attentively from the beginning to the end is able to solve it MOMENTARILY 
       if he/she does understand the meaning of the words.


2.  How many two-digit numbers contain the digit 5 ?

        Again, I will not solve it for you / instead of you.
        I will help you in other way, by separating the whole problem into the series of questions:

        a) how many two-digit numbers from 10 to 19 contain the digit 5 ?
        b) how many two-digit numbers from 20 to 29 contain the digit 5 ?
        c) how many two-digit numbers from 30 to 39 contain the digit 5 ?
           . . . . . . . .
        x) how many two-digit numbers from 90 to 99 contain the digit 5 ?        

        The rest is on you.


3.  How many digits are needed to write down all numbers from 1-100?

        Use the same approach: subdivide the BIG question into the series of smaller questions.


4.  the number of digits used to write all the page numbers in a book is 35. How many pages has the book?

        First think about pages from 1 to 9.

        Then think on pages from 10 to 19.


5.  How many whole numbers from 1 to 100 contain the digit 2 exactly once?

        Apply the same approach as in n.2.