Is 18 an irrational number

NO!!

Irrational Numbers are those where the decimal digits go to infinity AND there is no regular order in the decimal digits.

pi = 3.141592.... is the most well known irrational number.

as are the 'Square roots' od prime numbers;-#

sqrt(2) = 1.414213....

sqrt(3) = 1.73205...

sqrt(5) = 2.23606...

However,

such as numbers as

3.3333..... is NOT irrational, because the decimals are in regular order.

Similarly

2.373737....

or

5 .678678678....

Are NOT irrational because the decimals are in a regular order.

NB '....''' at the end of a decimal is the mathematical indication of recurring to infinity.