Manoj was a very careless driver, so his owner Ganpat gave him an offer that he will get an incentive of Rs30 for every bottle-box he delivered without breaking it and he will be charged Rs 90 for every bottle-box he broke.Ganpat gave Manoj 100 bottles-box to deliver. After delivery Ganpat paid Manoj Rs 2400.
How many bottles-box did Manoj break?