Q 7. CSAT 2020
How many zeroes are there at the end of the following product? 1 * 5 * 10 * 15 * 20 * 25 * 30 * 35 * 40 * 45 * 50 * 55 * 60
Zeros at the end means multiple of 10, i.e. (2 × 5).
• Now, 1 × 5 × 10 × 15 × 20 × 25 × 30 × 35 × 40 × 45 × 50 × 55 × 60 = 1 × 5 × (5 × 2) × (5 × 3) × (5 × 22) × (5 × 5) × (5 × 3 × 2) × (5 × 7) × (5 × 23) × (5 × 9) × (5 × 5 × 2) × (5 × 11) × (5 × 22 × 3)
• Here, number of 2s = 10
• And number of 5s = 14
• The lesser of the two will determine the number of zeros.
• Hence, there will be 10 zeros at the end in the given expression.
Q 8. CSAT 2020
Let XYZ be a three-digit number, where (X + Y + Z) is not a multiple of 3. Then (XYZ + YZX + ZXY) is not divisible by
• XYZ is a three-digit number.
• So, we can write this as: 100X + 10Y + Z (as X is at hundreds place and Y is at tens place)
• So, XYZ + YZX + ZXY = (100X + 10Y + Z) + (100Y + 10Z + X) + (100Z + 10X + Y) = 111 (X + Y + Z)
• Hence, it is divisible by (X + Y + Z).
• 111 is divisible by 3, as well as 37.
• So, the expression XYZ + YZX + ZXY is not divisible by 9.
Q 14. CSAT 2020
One page is torn from a booklet whose pages are numbered in the usual manner starting from the first page as 1. The sum of the numbers on the remaining pages is 195. The torn page contains which of the following numbers?
After tearing away the page, we get a sum of 195 from remaining page numbers. Since, numbers are in sequence, we use arithmetic progression.
Sum of consecutive numbers from 1 to n = n(n+1)/2 = 195 or n(n+1)= 390 with one page torn.
If we add the digits on torn page, we will get a number more than 390.
For n to be an integer, the minimum possible value of n(n+1) over 390 is received when we put n = 20
So, n(n+1) = 20 × 21 = 420
So, sum of 20 pages = n(n+1)/2 = (20 × 21)/2 = 210
Hence, sum of the two numbers on the torn page = 210 – 195 = 15
Only option b gives 15 as sum.
Hence, 7,8 is the answer.
Q 35. CSAT 2020
A is 16th from the left end in a row of boys and V is 18th from the right end. G is 11th from A towards the right and 3rd from V towards the right end. How many boys are there in the row?
• G is 3rd from V towards the right, so position of G = 18 – 3 = 15th from right end.
• G is 11th from A towards the right, so position of G = 16 + 11 = 27th from left end.
• Hence, total number of boys in the row = 27 + 15 – 1 = 41 (we subtract one because G has been counted twice)
Q 53. CSAT 2020
The recurring decimal representation 1-272727... is equivalent to
• Let x = 1.272727….
Then 100x = 127.272727….
So, 100x – x = (127.272727….) – (1.272727….) = 126
Or x = 126/99 = 14/11