SSC CGL/CHSL 2015:- QUANTATIVE APTITUDE

Number System tips and tricks

1)If you have to find the square of numbers ending with ‘5′.

e.g 25 * 25. Find the square of the units digit (which is 5) = 25. Write this down.
Then take the tenths digit (2 in this case) and increment it by 1 (therefore, 2 becomes 3). Now multiply 2 with 3 = 6.
Write ‘6′ before 25 and you get the answer = 625.

For instance, if you have to find the square of 45. 45 * 45.
The square of the units digit =- 25
Increment 4 by 1. It will give you ‘5′. Now multiply 4 * 5 = 20. Write 20 before 25. The answer is 2025.

In the case of numbers like 125. The rule applies without an iota of difference.
The square of the units digit = 25.
Increment 12 by 1. It will give you 13. Now multiply 12*13 = 156. Write 156 before 25. The answer is 15625.

2)This rule applies to numbers whose units digits add up to ten (2 and 8 in this case) and the tenth digits are same.
If you have a pair of numbers like 22 and 28 and you need to multiply the two of them.

Step 1:
Multiply the unit digits
2 * 8 = 16

Step 2:
Increment the tenth digit by 1.
2 + 1 = 3

Step 3:
Multiply the incremented number with the original tenth digit number
3 * 2 = 6

Step 4:
Concatenate the results got from step 1 and step 3
6 and 16 => 616

Note: Remember to put the products of unit digits of the given numbers in the unit and tenth digit of your result.

Example 2: 36 * 34 = ?

Step 1: Multiply the unit digits
6 * 4 = 24

Step 2: Increment the tenth digit by 1.
3 + 1 = 4

Step 3: Multiply the incremented number with the original tenth digit number
4 * 3 = 12

Step 4: Concatenate the results got from step 1 and step 3
12 and 24 => 1224

Therefore, 36 * 34 = 1224

Example 3: 61 * 69 = ?

Step 1: Multiply the unit digits
1 * 9 = 9. In this case remember to write 9 as 09

Step 2: Increment the tenth digit by 1.
6 + 1 = 7

Step 3: Multiply the incremented number with the original tenth digit number
6 * 7 = 42

Step 4: Concatenate the results got from step 1 and step 3
42 and 09 => 4209 (Since, we have to put the product of the unit digits only in the unit and tenth digit of the result)

Therefore, 61 * 69 = 4209

Example 4: 127 * 123 = ?

Step 1: Multiply the unit digits
7 * 3 = 21

Step 2: Increment the tenth digit by 1.
12 + 1 = 13

Step 3: Multiply the incremented number with the original tenth digit number
12 * 13 = 156

Step 4: Concatenate the results got from step 1 and step 3
156 and 21 => 15621

Therefore, 127 * 123 = 15621