number systems Model Questions & Answers, Practice Test for ssc chsl tier 1
L.C.M. of $2/3, 4/9, 5/6$ is
Answer: (a)
LCM of $2/3, 4/9, 5/6$
${\text"LCM of " (2,4,5)}/{\text"HCF of" (3,9,6)} = 20/3$
I have a certain number of beads which lie between 600 and 900. If 2 beads are taken away the remainder can be equally divided among 3, 4, 5, 6, 7 or 12 boys. The number of beads I have
Answer: (b)
LCM of the numbers = 420.
Hence there must be (420 × 2) + 2 = 842 beads.
The integers 1, 2, ...., 40 are written on a blackboard. The following operation is then repeated 39 times; In each repetition, any two numbers, say a and b, currently on the blackboard are erased and a new number a + b – 1 is written. What will be the number left on the board at the end?
Answer: (a)
1 + 2 + 3 + ..... + 40 = ${40 × 41}/2 = 820$
Since at each time any two numbers a and b are erased and a single new number (a + b – 1) is writen.
Hence, each one is subtracted and this process is repeated 39 times.
Therefore, number left on the board at the end = 820 – 39 = 781.
If all the numbers from 501 to 700 are written, what is the total number of times does the digit 6 appear ?
Answer: (b)
For number between 600 to 700 :
Number of 6 at the units place = 10
Number of 6 at the tens place = 10
Number of 6 at the hundredth place = 100
For number between 501 to 599 :
Number of 6 at the units place = 10
Number of 6 at the tens place = 10
Hence, total number of 6 between (501 – 700)
= 10 + 10 + 100 + 10 + 10 = 140
The sum of three prime numbers is 100. If one of them exceeds another by 36, then one of the numbers is
Answer: (d)
Let the three prime numbers be x, y, y + 36
x + y + y + 36 = 100 ⇒ x + 2y = 64
2y is an even number always
We know that
Even + even = even or odd + odd = even
So x has to be even to satisfy x + 2y = 64
The only even prime on is 2
Put x = 2 ⇒ 2y = 62 ⇒ Or y = 31
So the numbers are 2, 31, 67
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