21 : 81 : : 42 : ______

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625

648

640

164

answer is D.

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Detailed Solution

1296

Reason: map $ab \rightarrow (a+b)^4$ $ab$ $\to$ $($ $a$ $+$ $b$ $)$ $4$ .
For $21$ $21$ : $(2+1)^4=3^4=81$ $($ $2$ $+$ $1$ $)$ $4$ $=$ $3$ $4$ $=$ $81$ .
For $42$ $42$ : $(4+2)^4=6^4=1296$ $($ $4$ $+$ $2$ $)$ $4$ $=$ $6$ $4$ $=$ $1296$ .

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