Counting by 3 mean add three to previous number or we can say that
jump by 3 numbers. Here each number is three more than the previous
number. In counting by three's, if the first number is **an odd** number then the next number by adding three will be **an even** number. And if the first number is **an even** number then the next number will be **an odd** number. So counting by 3's gives you alternatively odd and even numbers.

**Example : Counting by three's**

If first number is even, **4** ; 4 + 3 = **7**; 7 + 3 = **10**; 10+ 3 = **13**; 13 + 3 = **16**; 16 + 3 = **19** and so on.

So if the 1st number is 4 then after counting by three's we will get **4, 7, 10, 13, 16, 19,...**

To find the next number by counting by three's, add 3 to the given number.

**Example 1 :** Find the next number of 18 after counting it by three's.

**Solution :** Add 3 to 18.

18 + 3 = 21.

So next number will be **21**.

**Example 2 :** Observe the following pattern and write the next two numbers.

8,11,14,17,20,____,_____

**Solution :**
By observing the above pattern, we can see that each number is 3 more
than the previous number.The first number is 8, next is 8 + 3 = 11; 11 +
3=14; 14 +3 = 17 and 17 + 3 = 20. So the next number after 20 is
obtained by adding 3 to 20 so the number is 20 + 3 = 23 and then 23 + 3 =
26 . 8,11,14,17,20,**23,26**.

**Example 3 :** Find the missing numbers using counting by three's. 43,___,____,____,____,58.

**Solution :** As we have to find the missing number by 3's so add 3 to each number.

1st missing number = 43 + 3 = 46

2nd missing number = 46 + 3 = 49

3rd missing number = 49 + 3= 52

4th missing number = 52 + 3 = 55

So the numbers are 34,__ 46,49,52,55__, 44.

**Practice**

Q.1 Find the missing numbers.

1) 13,16,___,___,___,___,___,34

2) 21,24,___,___,33,___,___,42

3) 51,___,57,60,___,___,___,72

4) 84,___, 90,93,___,___,___

5) ,___,20,23,___,29,___

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