Given: ²⁰C_r = ²⁰C_r–10 Need to find: Value of ¹⁷C_r We know, one of the property of combination is: If ⁿC_r = ⁿC_t, then, (i) r = t OR (ii) r + t = n We can’t apply the property (i) here. So we are going to use property (ii) ²⁰C_r = ²⁰C_r–10 By the property (ii), ⇒ r + r – 10 = 20 ⇒ 2r = 30 ⇒ r = 15 Therefore, ¹⁷C₁₅ = 136.