Use this number line: 1 2 3 4 5 6 7 8 9 10

Look what numbers are from on the left and the right from the missing number: 1 2 3 ? 5 6 7. Find these numbers in the line and determine what digit is between them. It will be the missing number.  

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A long time ago, people didn't have the knowledge that we have now. There were no shops and money, schools and even kindergartens. There was no one to teach them, and they learned everything from their own experience.

At first, people used their body parts to determine the amount of something. To indicate the number of two objects, they said "as many as I have hands". Denoting four items, they used the expression "as many as a dog has legs". But these phrases are so long. While explaining how many objects you need, someone will fall asleep. There should be short and clear signs for everyone.

Over time, people invented numbers.

Now all over the world, we use the same system for naming quantities. If you are a tourist and come to Singapore from Australia, the numbers in the store on the price tags will be the same as in your native country. Maybe the cost will be different, but the numbers that denote it are the same. We indicate the number of objects less than ten using a digit from 1 to 9. They look like this: 

1 - one 2 - two 3 - three 4 - four 5 - five

6 - six 7 - seven 8 - eight 9 - nine

We write the number 10 using two digits - 1 and 0 (zero). The number 1 shows the number of tens. And zero means that something is missing, as it doesn’t denote any object and is the beginning of the counting.

Remember the order of numbers from 1 to 10. It never changes: 1_2_3_4_5_6_7_8_9_10. Each number in this line has its place. If you learn this sequence by heart, you can always find which number is missing in the line and put it in its place.

Practice solving the following tasks.

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