Edit: It will take 12 steps together and 12 and a half steps separately instead of 7, 7and a half steps.
Mistake being regretted.

Answer lies in number of steps taken to sell marbles.
When they sell marbles together it takes 7 steps to sell 60 marbles and they will earn 24 paise
But when they sell marbles separately first woman will take 10 steps to sell 30 marbles earning 10 paise and second will take 15 steps earning 15 paise, 25 paise in total. Average of two is 7.5.means 7 and a half step
So this half step account for an extra paisa.