Question

Charlie wants to buy a new house but he wants a house number that is divisible by both 4 and 5. Houses for sale are numbered 100 to 150. Which of these houses would he want to buy?

• A. 110, 120 or 130
• B. 120, 140 or 150
• C. 100, 120 or 140
• D. 130, 140 or 150

Answer 1

The correct option is C 100, 120 or 140

4 and 5 are relatively prime numbers as both numbers do not share any common factor.
Hence, the number divisible by both 4 and 5 signifies the divisibility by $4\times 5$ = $20$.
Numbers starting from 100 to 150, which are divisible by 20 are: 100, 120 and 140.
So, the required house numbers are 100, 120 or 140.