Distance between $A$ and $B$ is $72$ km. Two men started walking from $A$ and $B$ at the same time towards each other. The person who started from A travelled uniformly with average speed $4$ kmph. While the other man travelled with varying speeds as follows: In first hour his speed was $2$ kmph. in the second hour it was $2.5$ kmph, in the third hour it was $3$ kmph, and so on. When will they meet each other?
1. $7$ hours
2. $10$ hours
3. $35$ km from $A$
4. midway between $A & B$