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 139 by the denominator 40 (which you can enter in any calculator):
139 (numerator) ÷ 40 (denominator) = 3.48
And finally, you get 3.48 as your answer when you convert 2 59/40 (or 139/40) to a decimal.