Divisibility Problem: Finding Numbers Divisible by 2, 4, and 5 Between 19-29

LCM Applications with Range Constraints

A nursery distributes an equal number of flowers every day so that no flowers are left undistributed in the field.

On the first day they were divided into pairs.

On the second day they were divided into 4 equal groups.

On the third day they were divided into 5 groups.

It is known that the number of flowers is greater than 19 and less than 29.

Calculate the number of flowers that are distributed in the nursery over the three days.

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Step-by-step written solution

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1

Understand the problem

A nursery distributes an equal number of flowers every day so that no flowers are left undistributed in the field.

On the first day they were divided into pairs.

On the second day they were divided into 4 equal groups.

On the third day they were divided into 5 groups.

It is known that the number of flowers is greater than 19 and less than 29.

Calculate the number of flowers that are distributed in the nursery over the three days.

2

Step-by-step solution

To solve the problem, we first determine the LCM of the numbers 2, 4, and 5:

  • The prime factors of 2 are 22.
  • The prime factors of 4 are 222^2.
  • The prime factors of 5 are 55.

The LCM is determined by taking the highest power of each prime number: 222^2 and 55, so LCM(2,4,5)=22×5=20\text{LCM}(2, 4, 5) = 2^2 \times 5 = 20.

Next, we consider numbers between 19 and 29: 20, 21, 22, 23, 24, 25, 26, 27, 28, 29.
Among these, only 20 is divisible by 20.

Therefore, the number of flowers is 2020.

So, the solution to the problem is: Number of flowers=20\text{Number of flowers} = 20.

3

Final Answer

20 20

Key Points to Remember

Essential concepts to master this topic
  • Rule: Find LCM by taking highest power of each prime factor
  • Technique: LCM(2, 4, 5) = 2² × 5 = 20
  • Check: Verify 20 ÷ 2 = 10, 20 ÷ 4 = 5, 20 ÷ 5 = 4 (all whole numbers) ✓

Common Mistakes

Avoid these frequent errors
  • Finding LCM incorrectly by multiplying all numbers together
    Don't calculate LCM(2, 4, 5) as 2 × 4 × 5 = 40! This ignores that 4 = 2² already contains factor 2, giving an unnecessarily large result. Always find prime factorizations first: LCM = 2² × 5 = 20.

Practice Quiz

Test your knowledge with interactive questions

Will a number divisible by 6 necessarily be divisible by 3?

FAQ

Everything you need to know about this question

Why do we need to find the LCM instead of just checking each number?

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The LCM (Least Common Multiple) gives us the smallest number that divides evenly by 2, 4, and 5. Since flowers must be distributed with no leftovers each day, we need a multiple of all three numbers!

How do I find the LCM when numbers share factors?

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Look at prime factorizations: 2 = 2¹, 4 = 2², 5 = 5¹. Take the highest power of each prime. Since 4 already contains 2², don't count the 2 from '2' separately!

What if there were multiple numbers in the range 19-29 that worked?

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We'd choose the smallest one since LCM gives the least common multiple. But here, only 20 works because the next multiple would be 40, which exceeds our range.

Can I just test each number from 20 to 28 by division?

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Yes! That's a valid approach. Check if each number divides evenly by 2, 4, and 5. But finding the LCM first is more efficient and helps you understand the mathematical structure.

Why isn't 24 the answer since it's divisible by 2 and 4?

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  • 24 ÷ 2 = 12 ✓
  • 24 ÷ 4 = 6 ✓
  • 24 ÷ 5 = 4.8 ✗

Since 24 ÷ 5 gives a decimal, there would be leftover flowers on day 3, violating the problem conditions.

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