Arithmetic aptitude :: HCF and LCM :: General Questions

• HCF and LCM - General Questions
• HCF and LCM - Important Formulas and Facts

Factors and Multiples:
If number x divided another number y exactly, we say that x is a factor of y.
and y is called a multiple of x.
Highest Common Factor (H.C.F.) or Greatest Common Measure (G.C.M.) or Greatest Common Divisor (G.C.D.):
The H.C.F. of two or more than two numbers is the greatest number that divides them exactly.
Mehods To Find HCF
Factorization Method:
Step 1) Find the prime factors of each number
Step 2) Find the product of all common prime factors, which is the HCF of given numbers
e.g. To find the HCF of 14 and 8
14 = 2 x 7
8 = 2 x 2 x 2
HCF = 2
Division Method:
To find the H.C.F. of two given numbers
Step 1)Divide the larger by smaller one.
Step 2)Divide the divisor by the remainder.
Step 3)Repeat the process of dividing the preceding number by the remainder last obtained till zero is obtained as remainder.
The last divisor is the required H.C.F.
Finding the H.C.F. of more than two numbers:
Step 1)H.C.F. of [(H.C.F. of any two) and (the third number)] gives the H.C.F. of three given number.
Similarly, we can find the H.C.F. of more than three numbers.
Least Common Multiple (L.C.M.):
LCM is the least number which is exactly divisible by each one of the given numbers.
Methods to find LCM
Factorization Method:
Step 1)Find prime number factors of each given number.
Step 2)Find the product of highest powers of all the prime number factors, which gives the LCM of given numbers
e.g. To find the LCM of 14 and 8
14 = 2 x 7
8 = 2 X 2 x 2 = 2 ³
LCM = 2 ³ x 7 = 56
Division Method (short-cut):
Step 1)Arrange the given numbers in a rwo in any order.
Step 2)Divide by a number which divided exactly at least two of the given numbers and carry forward the numbers which are not divisible.
Step 3)Repeat the above process till no two of the numbers are divisible by the same number except 1.
LCM = The product of the divisors and the undivided numbers
Product of two numbers = (HCF x L.C.M) of two numbers.
Co-primes: Co-primes means, if their H.C.F. is 1.
HCF and LCM of Fractions:
HCF = (HCF of Numerators) / (LCM of Denominators)
LCM = (LCM of Numerators) / (HCF of Denominators)
H.C.F. and L.C.M. of Decimal Fractions:
In a given numbers,
Step 1)Make the same number of decimal places by annexing zeros in some numbers, if necessary.
Step 2)Considering these numbers without decimal point, find H.C.F. or L.C.M.
Step 3)In the result, mark off as many decimal places as are there in each of the given numbers.
Comparison of Fractions:
Step 1)Find the L.C.M. of the denominators of the given fractions.
Step 2)Convert each of the fractions into an equivalent fraction with L.C.M as the denominator, by multiplying both the numerator and denominator by the same number. The resultant fraction with the greatest numerator is the greatest.

Which of the following is a pair of co-primes ?

Answer:

Explanation:

Two numbers are said to be co-prime if their H.C.F. is 1. from the given options H.C.F. of 18 and 25 is 1.
So, they are co-primes.

The H.C.F. of 204, 1190 and 1445 is :

Answer:

Explanation:

204 = 2² x 3 x 17

1190 = 2 x 5 x 7 x 17

1445 = 5 x 17²

HCF = 17

H.C.F. of 4 x 27 x 3125, 8 x y x 25 x 7 & 16 x 81 x 5 x 11 x 49 is :

Answer:

Explanation:

Given,

4 x 27 x 3125 : 2² x 3³ x 5⁵

8 x y x 25 x 7: 2³ x 5² x 7 x y

16 x 81 x 5 x 11 x 49: 2⁴ x 3⁴ x 7² x 5 x 11

HCF of all three is common of three As we have 2², 5, y(as the third number is given two numbers extra than in first ,assume second number will one number extra than the first)

 y can be multiples of 3 ,either 9 or 27 as the highest power of 3 among the two numbers is 3³

If y = 9 then HCF = 2² X 5 X 3² = 180

If Y = 27 then HCF = 2² X 3³ X 5 = 540

So ,as 540 is not given as option here the answer will be 180

A number a is said to be perfect if the sum of all its divisors (excluding n itself) is equal to n. An example of perfect number is :

Answer:

Explanation:

Let the number be n
Divisors of the number:
6: 1, 2, 3 and its sum 1+2+3 = 6
9: 1, 3, 9 and its sum 1+3+9 = 13
15: 1, 3, 5, 15 and its sum 1+3+5+15 = 24
21: 1, 3, 7, 21 and its sum 1+3+7+21 = 32
So the n is 6

Which of the following has most number of divisors ?

Answer:

Explanation:

99 = 1 x 3 x 3 x 11;

101 = 1 x 101;

176 = 1 x 2 x 2 x 2 x 2 x 11;

182 = 1 x 2 x 7 x 13.

So, divisors of 99 arc 1, 3, 9,11, 33 and 99;
divisors of 101 are- 1 and 101;

divisors of 176 are 1, 2, 4, 8, 16, 22, 44, 88 and 176;

divisors of 182 are 1, 2, 7, 13, 14, 26, 91 and 182. Hence, 176 has the most number of divisors.

• HCF and LCM - General Questions