The number 5

Established fact In maths and on the calendar

• 5 is prime: its only divisors are 1 and 5.
• 5 is a Fibonacci number, part of the sequence where each number is the sum of the two before it.
• 5 is the long side of the 3-4-5 right triangle (3² + 4² = 5²).
• 5 is a side of the 5-12-13 right triangle (5² + 12² = 13²).
• Every month has a 5th, so it comes round twelve times a year.