10 lemons sold at Tk 50 with 50% loss. If 6 lemons sold at Tk 50, profit/loss?

A 50% loss on Tk 50 for 10 lemons means their cost is Tk 100, i.e. Tk 10 each, so 6 lemons cost Tk 60. Selling those 6 for Tk 50 is a loss of Tk 10 on Tk 60, which is $\dfrac{10}{60}\times100 = \dfrac{50}{3}\%$ loss.