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6.Permutation and Combination
normal
$^{20}C_1 + 3 ^{20}C_2 + 3 ^{20}C_3 + ^{20}C_4$ is equal to-
A
$^{20}C_4$
B
$2. ^{21}C_4$
C
$2. ^{22}C_4$
D
$^{23}C_4$
Solution
$({\,^{20}}{{\rm{C}}_1} + 2{\,^{20}}{{\rm{C}}_2} + {\,^{20}}{{\rm{C}}_3}{\rm{)}} + {{\rm{(}}^{20}}{{\rm{C}}_2} + 2{\,^{20}}{{\rm{C}}_3} + {\,^{20}}{{\rm{C}}_4}{\rm{)}}$
$ \Rightarrow {\,^{22}}{C_3} + {\,^{22}}{C_4} = {\,^{23}}{C_4}$
Standard 11
Mathematics