To find the fourth proportional to 1/3, 1/5, and 1/7, we can set up a proportion:

1/3 : 1/5 = 1/7 : x

To solve for x, we can cross-multiply:

(1/3)(x) = (1/5)(1/7)

Multiplying both sides by 3:

x = (1/5)(1/7)(3)

Simplifying:

x = 3/35

Therefore, the fourth proportional to 1/3, 1/5, and 1/7 is 3/35.