Two digit numbers can be represented by groups of ten and ones.
 

In the number 24.  The 2 is in the tens place and the 4 is in the ones place.
In other words, there are 2 groups of ten and 4 ones.  Here are some additional examples.

  What do I mean by groups of ten?

Groups of ten are also called rods.    

                       Notice that there are 10 small cubes that make up one rod.  That is why it is called a group of ten.

Here is a one or unit cube.  Ten of the unit cubes make up a group of ten or rod.

To represent a number using rods and  unit cubes, you look at the numbers in the ones and tens place.

If you were to check your answer for number 15, you could count every block, but that would take a lot of time.  You could say "ten" for the group of ten and then just keep counting. "10.......11,12,13,14,15." 

The same goes for 23 and 47.

You could say "10,20,.......21,22,23" 

For 47 it would be "10,20,30,40,.........41,42,43,44,45,46,47.

 Let's try some:

How many rods and cubes would you use to represent the number 52?

Number	Rods (Groups of ten)	Units (Ones)
15
23
40
62

What happens if you have a three digit number?

You would use a flat or 100-block.

A flat is made up of 10 rods

10 rods make a flat.

Number	Flat (100-block)	Rods (Groups of ten)	Unit (ones)
132
341
527

10	make a	ten (rod)
10	make a	one hundred (flat)
10	make a	1,000!