What Are Some Facts About the Number 55?

By Staff WriterLast Updated Apr 14, 2020 8:23:34 AM ET

The number 55 is the largest Fibonacci number to be both triangular and semi-prime. Also, any odd number between 90 and 110 multiplied by 55 would result in a palindrome numeral. The number 55 is also the atomic number for caesium.


Fibonacci numbers follow a integer sequence, (1, 1 ,2 ,3 ,5, 8...) by adding the previous number to the next number in sequence. Following this pattern, 55 is tenth number in the Fibonacci sequence (21+34). It is a figurate number, meaning that the same number of equally spaced dots can be arranged in the shape of a regular polygon.

The number 55 is also semi-prime. A prime number is divisible only by 1 and itself, while a semi-prime (also called a bi-prime) is a product of two prime numbers.

Multiplying any odd number between 90 and 110 by 55 results in a palindrome numeral, which can be written the same backwards and forwards. For example, 91 multiplied by 55 equals 5005.

In science, Caesium is a soft, yet highly reactive metal whose atomic number is 55. The atomic number is the number of protons in the nucleus of an atom. This number determines its place in the periodic table of elements.