Add: 2/5 + 3/7 = 2 · 7/5 · 7 + 3 · 5/7 · 5 = 14/35 + 15/35 = 14 + 15/35 = 29/35.
It is suitable to adjust both fractions to a common (equal, identical) denominator for adding, subtracting, and comparing fractions. The common denominator you can calculate as the least common multiple of both denominators - LCM(5, 7) = 35. In practice, it is enough to find the common denominator (not necessarily the lowest) by multiplying the denominators: 5 × 7 = 35. In the following intermediate step, it cannot further simplify the fraction result by canceling.
In other words - two fifths plus three sevenths is twenty-nine thirty-fifths.