RRB NTPC - Number System : Aptitude Test (200 Questions with Explanation)

1.

Find the LCM of 25 , 20?

   A.) 100
   B.) 320
   C.) 400
   D.) 300

Answer: Option 'A'

The fastest way to find the correct answer is by elimination rule :
Check whether each of the given choices is divisble by both the numbers. If any choice doesn’t get divided by both the numbers, then you can eliminate that particular choice and move to the next number. Answer will be the Number that is exactly divisble by both given numbers.
Please Note : There may be a scenario when two or more numbers in the choice will be exactly divisible by both the numbers, in that case , please choose the lowest among the numbers.

2.

The difference between a two digit number and the number obtained by interchanging two digits of the number is 9.The sum of the two digits of the number is 15. What is product of the two digits of two digit number?

   A.) 36
   B.) 56
   C.) 54
   D.) 72

Answer: Option 'B'

Sum of the digits is 15, this can happen only when the digits are 7 and 8.
The number is 87, interchanging digits we get 78,
Difference between these two numbers = 9
Product of these two digits = 7 x 8 = 56

3.

The integers 32489 and 33714, when divided by a three digit number "n", leave the same remainder. What is the value of "n"?

   A.) 225
   B.) 215
   C.) 245
   D.) 235

Answer: Option 'C'

Given two numbers 32489 and 33714.
Let r be the remainder we get, when we divide these both numbers by n, then
32489 = qn + r and 33714 = pn + r,
Where p and q are some integers.
Subtracting these two equations, we get
1225 = (p-q) x n.
We can write 1225 as 5 x 245.
Hence 245 is the answer.

4.

What is the sum of all two digit numbers? 

   A.) 4900
   B.) 4895
   C.) 4905
   D.) None of these

Answer: Option 'C'

10 + 11 + 12 + 13 + 14 + 15 + - - - - - - - - + 97 + 98 + 99 + 100 
Sn = n/2[a + l] 
= 90/2[10 + 99] 
= 45[109] 
= 4905.

5.

If a and b are odd numbers, then which of the following is even?

   A.) a + b
   B.) a + b + 1
   C.) ab + 2
   D.) ab

Answer: Option 'A'

The sum of two odd number is even. So, a + b is even.

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