In the "1s" place,
1₅ + 3₅ = 4₅
In the "5s" place,
2₅ + 4₅ = (6)₅ = (5 + 1)₅ = 10₅ + 1₅ = 11₅
In the "25s" place, we carry the leading 1 from the "5s" place,
3₅ + 3₅ + 1₅ = (7)₅ = (5 + 2)₅ = 10₅ + 2₅ = 12₅
In the "125s" place, we carry over the 1 again,
4₅ + 1₅ = (5)₅ = (5 + 0)₅ = 10₅ + 0₅ = 10₅
Putting everything together,
4321₅ + 343₅ = 10214₅
