5th Grade Multiplication | Concepts and Sample Problems 1/4 x 1/2 = 1/8. Students learn to multiply two fractions by multiplying the numerators of the two fractions for the new numerator and multiplying the two denominators for the new denominator. Explain the logic behind the common presentation of this rule: a/b x c/d = ac/bd. 4/5 x 2/3 = (4 x 2) ÷ (5 x 3) = 8/15.