91 (number) facts for kids
Ninety-one is a number that comes right after ninety and before ninety-two. It is an odd number, which means you can't divide it evenly by two.
Ninety-one is special because it can be divided by a few numbers without leaving a remainder. These numbers are 1, 7, 13, and 91 itself.
Contents
What Makes 91 Special?
A Composite Number
Ninety-one is a composite number. This means it has more than two factors (numbers that divide into it evenly). For example, 7 and 13 are factors of 91 because 7 multiplied by 13 equals 91. A prime number only has two factors: 1 and itself.
A Semiprime Number
Because 91 is the result of multiplying two prime numbers (7 and 13), it's also called a semiprime number. Think of it as a special kind of composite number made from just two prime building blocks.
Triangular and Pyramidal
Ninety-one is a triangular number. You can make a triangle shape with 91 dots! It's the sum of all whole numbers from 1 to 13 (1 + 2 + 3 + ... + 13 = 91).
It's also a square pyramidal number. This means you can stack squares on top of each other to form a pyramid, and the total number of blocks would be 91. It's the sum of the first six square numbers (1² + 2² + 3² + 4² + 5² + 6² = 1 + 4 + 9 + 16 + 25 + 36 = 91).
Where You Might See 91
In Sports
In some sports, like American football, the number 91 might be worn by a player as their jersey number. Different numbers often belong to players in specific positions.
In Time and Dates
The 91st day of the year falls in early April (April 1st in a common year, March 31st in a leap year). This is just over three months into the year.
In Science
The element Protactinium has an atomic number of 91. This means it has 91 protons in the nucleus of each of its atoms.
