When 4 coaches (m each) are attached with engine (2m)
According to question, $P=K·6mgv$          .......(1)

(constant power),      (K being proportionally constant)

Since resistive force is proportional to weight
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Now if 12 coaches are attached

$P=K·14mg·v_1$            ......(2)

So by equation (1) and (2)

$6mgv=14mg{v}_1 \Rightarrow v_1=\frac{6}{14}\times v$

$=\frac{6}{14}\times 20=\frac{6\times 10}{7}=\frac{60}{7}\Rightarrow v_1=8.5 \mathrm{m} {\mathrm{s}}^{-1}$

Similarly for 6 coaches, $K·6mgv = K·8mg{v}_2$

$\Rightarrow v_2=\frac{6}{8}\times 20=\frac{3}{4}\times 20=15 \mathrm{m} {\mathrm{s}}^{-1}$