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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 PDF (31-Jan-2021) (Shift 1)
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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  2. There are four table clocks. They ring every 10 min, 15 min, 20 min and 25 min respectively. If they all ring together at 10 a.m., then at what time will they ring together again?
  3. Find the HCF of $12 \times 15, 15 \times 21, 21 \times 12$
  4. How many numbers less than 10000 are there which are exactly divisible by 21, 35 and 63?
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Important Questions from LCM and HCF

  1. The HCF and LCM of two numbers are 12 and 72, respectively. If the ratio of the two numbers is 2 ∶ 3, then the larger of the two numbers is:

  2. Find the greatest number that will divide 43, 91 and 183 so as to leave the same remainder in each case.

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  4. What is the least number which when divided by 12,20 and 24 leaves in each case a remainder of 8?

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