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 122 by the denominator 22 (which you can enter in any calculator):
122 (numerator) ÷ 22 (denominator) = 5.55
And finally, you get 5.55 as your answer when you convert 2 78/22 (or 122/22) to a decimal.