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Question

The LCM of two numbers is 721, and the numbers are in the ratio of 1 : 7. What is the sum of the numbers?

This question was previously asked in
RRB NTPC 2019 CBT 1 Question Paper (8-Mar-2021) (Shift 2)
The correct answer is
824

Ratio Numbers LCM Calculation

Let the two numbers be represented based on their given ratio of 1:7. We can express these numbers as $k$ and $7k$, where $k$ is a common factor.

LCM Relationship Explained

The Least Common Multiple (LCM) of two numbers $a$ and $b$ is the smallest positive integer that is divisible by both $a$ and $b$. For numbers $k$ and $7k$, their LCM is $7k$. This is because $7k$ is divisible by $k$ (result is 7) and by $7k$ (result is 1).

Calculating the Common Factor

We are given that the LCM of the two numbers is 721. Using the relationship derived above:

$7k = 721$

To find the value of $k$, we divide both sides of the equation by 7:

$k = \frac{721}{7}$

$k = 103$

Determining the Numbers

Now that we have the value of $k$, we can find the two numbers:

  • First number: $k = 103$
  • Second number: $7k = 7 \times 103 = 721$

Sum of Numbers Calculation

The question asks for the sum of these two numbers. We add the numbers found:

Sum = $103 + 721$

Sum = $824$

Thus, the sum of the two numbers is 824.

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Important Questions from LCM and HCF

  1. Six bells begin to toll together and toll, respectively, at intervals of 3, 4, 6, 7, 8 and 12 seconds. After how many seconds, will they toll together again?

  2. A and B are two prime numbers such that A > B and their LCM is 209. The value of A 2 - B is:

  3. Find the least number which when divided by 12, 18, 24 and 30 leaves 4 as remainder in each case, but when divided by 7 leaves no remainder.

  4. Calculate the HCF of \(\frac{12}{5}\) \(\frac{14}{15}\)  and  \(\frac{16}{17}\) .

  5. Three numbers are in the proportion of 3 : 8 : 15 and their LCM is 8280. What is their HCF?

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