Two persons Choton and Billal started a business in which Choton invested Tk. 50000, Billal invested Tk. 80000, after 4 months Robiul joined them with a certain amount. At the end of the year, a total profit of Tk. 40000 was recorded. Robiul's share in the profit was Tk. 15000, then find Robiul's investment in the business.