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 40 by the denominator 78 (which you can enter in any calculator):
40 (numerator) ÷ 78 (denominator) = 0.51
And finally, you get 0.51 as your answer when you convert 40/78 to a decimal.
