Addition and subtraction of fractions with the same denominator: denominator unchanged, numerator addition and subtraction.

That is:

When two fractions with the same denominator are added, the original denominator is the denominator of sum, and the result of adding two molecules is the numerator of sum.

For example, 2/5+ 1/5=3/5.

When two fractions with the same denominator are subtracted, the original denominator is the denominator of the difference, and the numerator of the difference is the result of subtracting the minuend from the numerator.

For example, 5/7-3/7=2/7.