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 138 by the denominator 48 (which you can enter in any calculator):
138 (numerator) ÷ 48 (denominator) = 2.88
And finally, you get 2.88 as your answer when you convert 1 90/48 (or 138/48) to a decimal.