We know that if , then one of the following conditions need to be satisfied:

i. p = q

ii. n = p + q

Let us use condition (i),

⇒ 3r = r + 3

⇒ 2r = 3

⇒

⇒

⇒

We know that for a combination ⁿC_r,r≥0 and r should be an integer, which is not satisfies here,

So, the condition (ii) must be satisfied,

⇒ 15 = 3r + r + 3

⇒ 4r = 12

⇒

⇒ r = 4.

∴ The value of r is 4.