KarunMendiratta wrote:
Explanation:
We will use three key concepts to solve this question;
1. When we divide 25! by 107, the last 7 digits would be the remainder
ex: if we divide 5! by 100, the last 2 digits would be the remainder
5!=(5)(4)(3)(2)(1)=120
Remainder = 20
2. In order to find the maximum power of any number (say a) in N! (say), divide N! by the increasing power of a till the numerator remains greater than the denominator
ex: to find number of 2's in 4!:
421+422
2+1=3
Therefore, number of 2's in 4! is 3
3. Cyclicity of 2, 3, 7 and 8 is 4 whereas cyclity of 4 and 9 is 2
i.e. the units digit of 2, 3, 7, and 8 repeat every 4 powers and units digit of 4 and 9 repeat every 2 powers
Coming back to the Question:
Find the number of 2's, 3's, 5's, 7's, 11's, 13's, 17's, 19's and 23's in 25! by using the second concept.
Number of 2's = 22
Number of 3's = 10
Number of 5's = 6
Number of 7's = 3
Number of 11's = 2
Number of 13's = 1
Number of 17's = 1
Number of 19's = 1
Number of 23's = 1
i.e. 25!=(222)(310)(56)(73)(112)(13)(17)(19)(23)
25!=(216)(310)(73)(112)(13)(17)(19)(23)(106)
Now, Units digit of:
(216) is 6
(310) is 9
(73) is 3
(112) is 1
(13) is 3
(17) is 7
(19) is 9
(23) is 3
So, 25!=(ab6)(cd9)(ef3)(gh1)(ij3)(kl7)(mn9)(op3)(106)=abcdefghijklmnop4(106)
Col. A: 4(106)
Col. B: 2(106)
Hence, option A
Hi
KarunMendiratta,
The solution makes sense, but I have a small doubt, why don't you consider primes in non-prime integers like 4( has 2 '2s'), 9( has 2 '3s'), and 15(one 3 and one 5) in the solution and that would change the number of exponents each prime number has in it. Could you please elaborate on that?