First the son and daughter cross the bridge together. (It takes them 3 minutes.) Then one of them - for instance, the son - returns to the parents. (Add 1 minute.) The father and mother cross the bridge together (10 minutes). The daughter comes back (3 minutes). The son and daughter cross the bridge again (3 minutes). Thus the total time is 3 + 1 + 10 + 3 + 3 = 20 minutes.