‌10Ci‌20Cm−i =‌10C0‌20Cm+‌10C1‌20Cm−1+‌10C2‌20Cm−2+...+‌10Cm⋅‌20C0 Coefficient of xm in the expansion of the product ‌(1+x)10(1+x)20 ‌=‌ Coefficient of ‌xm‌ in ‌(1+x)30 ‌=‌30Cm. Maximum value of ‌30Cm=‌30C15 ∴‌‌m=15