All items are grouped by 5. To count all groups, assign each one a number from the sequence:

5, 10, 15, 20, 25, 30, 35, 40, 45, 50, etc.

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Here is a stopwatch.

People use it in sports to calculate who reached the finish faster. We can notice the numbers on it are placed not in order from 1, but every 5: 0, 5, 10, 15, 20 to 60. It helps to save space, as instead of putting each number from 0 to 60, there are only 12 numbers on the dial. We call it skip-counting.

Skip-counting is a method we use not to count every object but groups of several objects. The significant point is that the groups should contain the same number of items. Only in this case we can use a certain number sequence.

When counting by one object, we apply an ordinary number sequence, in which each next number is one greater than the previous one: 1, 2, 3, 4, 5, 6, 7, 8, 9… 

When counting by pairs, we use numbers skipping 1:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20…

When counting by 3s, we call groups each third number:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 …

What numbers will we use to call groups of objects in this case? 

There are circles grouped in sets with 5 ones in each. To count them all, we can add the number 5 five times. 

5+5=10 (we added the number 5 two times)

10+5=15 (we added 5 the third time)

15+5=20 (here we added 5 the fourth time)

20+5=25 (we added 5 the last fifth time)

We got 25 circles in total.

By adding 5 each time, we get the number that is 5 greater: 5, 10, 15, 20, 25. We can use this sequence to count objects arranged in groups by 5. It is easy to remember, as each number in this sequence ends with 5 or 0. So to count numbers by 5s, call each 5 items the number in order: 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60… 

The last number you say will determine the total number of objects.  

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