Let's break down the information provided:

During the promotion, a customer can buy x donuts for the regular price of Tk. 15 and get 3 donuts free.
The donut price per dozen during the promotion is Tk 2 less than the normal donut price per dozen.
Let's denote the normal price per dozen as "P" (in Tk).

So, during the promotion:

Price for x donuts (including 3 free donuts) = Tk. 15
Price per dozen during the promotion = P - 2 Tk
Now, let's set up the equation for the price of x donuts during the promotion:

(x + 3) * (P - 2) = 15

Now, we need to find the value of x. We know that P is the price per dozen during the normal period.

To solve for x, we'll first express P in terms of x:

P = (15 / (x + 3)) + 2

Now, we need to find a value of x for which P is a multiple of 5 (to make the total price of x donuts equal to Tk. 15). We'll try different values of x:

If x = 15: P = (15 / (15 + 3)) + 2 = 1 + 2 = 3 The total price for 15 donuts (including 3 free) at Tk. 3 per dozen would be Tk. 15.
So, the value of x is indeed 15, and this satisfies the condition of the promotion.

Therefore, x = 15.