The sum of two-fifths and f is one-half. We write two-fifths as 2/5. The sum of two-fifths and f is written by adding f to two-fifths using the + sign: 2/5 + f one-half is written as 1/2 The word is means equals, so we set up an equation where 2/5 + f equal to 1/2 2/5 + f = 1/2