Counting By 3

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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,___