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Factorials & Power Cycles Questions and Answers

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21.

Find the unit digit of \(111^{111} \times 222^{222} \times 333^{333}\)

Answer: A

To find the unit digit of the expression, we only need to consider the unit digits of the bases. The problem is equivalent to finding the unit digit of the product \(1^{111} \times 2^{222} \times 3^{333}\).

Unit digit of \(1^{111}\) is always 1.

For \(2^{222}\), the power cycle of 2 is (2, 4, 8, 6), which repeats every 4 powers. We find the remainder of \(222 \div 4\), which is 2. So the unit digit is the second in the cycle, which is 4.

For \(3^{333}\), the power cycle of 3 is (3, 9, 7, 1), which repeats every 4 powers. We find the remainder of \(333 \div 4\), which is 1. So the unit digit is the first in the cycle, which is 3.

The unit digit of the entire product is the unit digit of \(1 \times 4 \times 3 = 12\), which is 2.

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23.

What is the unit digit of \(9^{99}\)?

Answer: D

The unit digits of powers of 9 alternate:

\[ 9,\ 1,\ 9,\ 1,\ \ldots \]

Odd exponents end in 9. Since 99 is odd, the unit digit is 9.

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24.

How many trailing zeros are in 1000! ?

Answer: B

We need to find the number of factors of 5 in 1000!. Using Legendre's formula: \(\lfloor 1000/5 \rfloor + \lfloor 1000/25 \rfloor + \lfloor 1000/125 \rfloor + \lfloor 1000/625 \rfloor = 200 + 40 + 8 + 1 = 249\). There are 249 trailing zeros.

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25.

Find the unit digit of \(25^{45} \times 56^{78} \times 89^{12}\).

Answer: A

To find the unit digit of the product, we can find the unit digit of each factor.

The unit digit of \(25^{45}\) is 5, as any power of a number ending in 5 will end in 5.

The unit digit of \(56^{78}\) is 6, as any power of a number ending in 6 will end in 6.

The unit digit of the product will be the unit digit of \(5 \times 6 = 30\), which is 0. We don't even need to calculate the third term's unit digit, as any number multiplied by a number ending in 0 will also end in 0.

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26.

Find the unit digit of \((1!)!\).

Answer: A

Work outward from the inner factorial.

\[ 1! = 1 \quad\Longrightarrow\quad (1!)! = 1! = 1 \]

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27.

What is the unit digit of \(8^{88}\)?

Answer: D

The power cycle for the unit digit of 8 is (8, 4, 2, 6), which has a length of 4. We need to find the remainder of the power \(88\) when divided by 4. Since 88 is a multiple of 4, the remainder is 0. A remainder of 0 corresponds to the last digit in the cycle, which is 6.

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28.

What is the highest power of 5 that divides the product \(1 \times 3 \times 5 \times ... \times 99\)?

Answer: B

We need to count the number of factors of 5 in the product of all odd numbers from 1 to 99. The multiples of 5 in this product are 5, 15, 25, 35, 45, 55, 65, 75, 85, 95.

Let's count the factors of 5 from each term:

5 = \(1 \times 5\) (one 5)

15 = \(3 \times 5\) (one 5)

25 = \(5 \times 5\) (two 5s)

35 = \(7 \times 5\) (one 5)

45 = \(9 \times 5\) (one 5)

55 = \(11 \times 5\) (one 5)

65 = \(13 \times 5\) (one 5)

75 = \(3 \times 25 = 3 \times 5 \times 5\) (two 5s)

85 = \(17 \times 5\) (one 5)

95 = \(19 \times 5\) (one 5)

Total number of factors of 5 is \(1+1+2+1+1+1+1+2+1+1 = 12\).

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29.

Find the unit digit of \(6^{6^{6}}\)

Answer: D

Every positive power of 6 ends in 6:

\[ 6,\ 36,\ 216,\ 1296,\ \ldots \]

So whatever the exponent \( 6^{6} \) works out to, the unit digit is 6 - the tower never has to be evaluated.

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30.

What is the remainder when \(1! + 2! + 3! + ... + 100!\) is divided by 8?

Answer: A

We need to find the remainder of the sum when divided by 8. Let's look at the remainders of the first few factorials. \(1! = 1\) (rem 1). \(2! = 2\) (rem 2). \(3! = 6\) (rem 6). \(4! = 24\). Since 24 is a multiple of 8, the remainder is 0. For any \(n \ge 4\), \(n!\) contains the factors 4 and 2, so it is a multiple of 8. The remainder for all subsequent factorials will be 0. We only need to sum the remainders of the first three factorials: \(1 + 2 + 6 = 9\). The remainder of \(9 \div 8\) is 1.

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Tags: Factorials Questions and Answers || Power Cycles MCQ Questions and Answers || Factorial and Power Cycle GK Questions || GK on Factorials || Quantitative Aptitude Factorial MCQ