Correct Answer:  850.

Explanation:

Let, the cost price is x Tk.

At 20% profit, the selling price = x + 20% of x = x+$\frac{x}{5}$ = $\frac{6x}{5}$

When cost price is increased by Tk. 50, then new cost price will be = (x + 50) Tk.

Again, if the selling price is also increased by Tk. 30, new selling price will be = ( $\frac{6x}{5}$+30) Tk

According to question,

( $\frac{6x}{5}$+30) – (x + 50) = (20% – $\frac{10}{3}$ %)(x + 50)

x = 850

Cost price is Tk. 850

Alternative:Let, the cost price be = x

According to the question,

=>($\frac{x\times \frac{120}{100}+30)-(x+50)}{5(x+50)}$=20%- $\frac{10}{3}$%

=>$\frac{6x+150-5x-250}{5(x+50)}$=$\frac{60-10}{300}$

=>$\frac{x-100}{5(x+50)}=\frac{50}{300}$

=> 6x-600

=>5x+250

x=850.