21? × 216.5 = 2112.4
21? × 216.5 = 2112.4 Correct Answer 5.9
$$\eqalign{ & {21^?} \times {21^{6.5}} = {21^{12.4}}.....\left \cr & \Rightarrow {21^{? + 6.5}} = {21^{12.4}} \cr & \Rightarrow ? + 6.5 = 12.4 \cr & \Rightarrow ? = 12.4 - 6.5 \cr & \Rightarrow ? = 5.9 \cr} $$