The distance between Town Y and Town Z is 7 km .

At the beginning , both Ali and Bob are at Town Y .

Ali walked from Town Y to Town Z at a constant speed .

After Ali had walked for 1 km , Bob discovered that Ali had left his belongs at Town Y .

Bob immediately ran after Ali at a constant speed of 4 km/h .

After handing over the belongings to Ali , Bob turned back and started running towards Town Y at the same speed .

Given that both Ali and Bob reached Town Z and Y respectively at the same time , what was Ali's speed ?