XAT, CAT MBA Entrance Math practice questions - Numbers, factorials, indices : Ascent Education XAT 2020

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You are here: Home   XAT, TANCET Prep Questions  Number Theory   Factorials, divisors Question 15

What is the highest power of 7 that divides 50!?

This question is from the topic Number Theory. It is a question on factorials and is about finding the highest power of a prime factor that will divide the factorial.

Question

What is the highest power of 7 that can divide 5000! without leaving a remainder? (5000! means factorial 5000)
  1. 4998
  2. 714
  3. 832
  4. 816
Correct Answer - 832. Choice (3)

Explanatory Answer

5000! = 5000 * 4999 * 4998 * 4997 * ............ 1.
Between 1 and 5000, there are numbers which will be divisible by 7, 72, 73 and 74.

There are 500/7 = 714 numbers that are exactly divisible by 7 between 1 and 5000.

So there are 714 Sevens contained in these numbers.

There are = 102 numbers that are exactly divisible by 49 between 1 and 5000.

These numbers are also a part of the 714 numbers. But we add it again because they are divisible by 72 and hence to account for the second 7 in these numbers.

There are  = 14 numbers that are divisible by 343 (i.e., 73) between 1 and 5000.

These numbers will be a part of the previous set 714 and 102 - however, we add these 14 numbers to account for the third seven in these numbers as these numbers are multiples of 343 or 73.

And finally, there are  = 2 numbers that are divisible by 2401 (i.e. 74).

Therefore, there will be a total of 714 + 102 + 14 + 2 = 832 sevens contained in 5000!
Hence the highest power of 7 that can divide 5000! without leaving a remainder is 832.

Correct answer choice (3)

Level of difficulty : Moderate to Difficult



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