Number System-Railway Maths

Total Questions: 50

21. When you reverse the digits of the number 14, the number increases by 27. How many other two-digit numbers increase by 27, when their digits are reversed? [NTPC CBT-1 22/01/2021 (Evening)]

Correct Answer: (d) 5
Solution:

Reverse of 14 = 41
Difference between 41 and 14 = 27
Five numbers of two digits increase by 27 when their digits are reversed.
25 ↔ 52 , 69 ↔ 96, 47 ↔ 74 41 ↔ 14 and 58 ↔ 58

22. A boy was set to multiply 495 by 36, but reading one of the digits in the questions erroneously, he obtained 16740 as his answer. Which digit did he read erroneously? [NTPC CBT-1 27/01/2021 (Evening)]

Correct Answer: (a) 9
Solution:

If we multiply 495 by 36 we get 17820 as result. But if boy read 6 in place of 9 he got 16740 as result So 9 is erroneously read as 6.

23. Assuming A = 1 , B = 2 and so on ......... Z = 26 find the value of the following equation. [NTPC CBT-1 12/02/2021 (Morning)]

(I² - C² × P/R) + 8

Correct Answer: (c) 81
Solution:

24. 24. Find the value of 1/(1 × 4) + 1/(4 × 7) + 1/(7×10) + .......+ 1 /(47×50) [NTPC CBT-1 16/02/2021 (Evening)]

Correct Answer: (c) 49/150
Solution:

25. If the sum of two numbers is r and their quotient is s/t then the numbers are. [NTPC CBT-1 17/02/2021 (Morning)]

Correct Answer: (b) (sr)/(s + t) and (tr)/(s + t)
Solution:

26. If the arithmetic geometric mean of two observations are 10 and 5, respectively, then find the sum of the squares of the observations. [NTPC CBT-1 11/03/2021 (Evening)]

Correct Answer: (d) 350
Solution:

Arithmetic mean = 10
Let the numbers be a and b
According to question,
a + b = 20
And,  √ab = 5 ⇒ ab = 25
Now (a + b)² = a² + b² + 2ab
⇒ 20² = a² + b² + 2 × 25
⇒ 400 = a² + b² + 50 ⇒ a² + b² = 350

27. Determine the integer n such that n/160 is less than 1/16 but more than 1/20. [NTPC CBT-1 11/03/2021 (Evening)]

Correct Answer: (d) 9
Solution:

28. What would be the highest value of X and Z in the given equation? [NTPC CBT-1 02/03/2021 (Evening)]

9 X7+8YZ+7Z1=2526

Correct Answer: (d) x = 3 , Z = 8
Solution:

29. If P is a prime number and P divides Q² , then P will NOT necessarily divide: [NTPC CBT-1 27/03/2021 (Morning)]

Correct Answer: (b) Q + 1
Solution:

If P is a prime number and P divides Q² then P will also divide Q So P will not divide the next number after Q
So Q + 1 is not divisible by P.

30. When 43 is divided by x, the remainder is x-5. If x is a natural number, how many solutions will x have? [NTPC CBT-1 23/07/2021 (Evening)]

Correct Answer: (c) 6
Solution: