A fraction is usually split into two parts: the first part is the number on top, called the numerator; and the second part is the number on the bottom, called the denominator. These are both separated by a line called the “divisor line”. We can use the division method help to solve this question: to get a decimal, simply divide the numerator 6840 by the denominator 55 (which you can enter in any calculator):
6840 (numerator) ÷ 55 (denominator) = 124.36
And finally, you get 124.36 as your answer when you convert 123 75/55 (or 6840/55) to a decimal.