Given $1.86 \mathrm{g}$

$\Rightarrow 1 \mathrm{mol} C_6{\mathrm{H}}_5{\mathrm{NH}}_2$ give $1 \mathrm{mol}$ 

$\therefore$ moles of ${\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{NH}}_2=$ moles of 

$\Rightarrow \frac{1.86}{93}=\frac{{\mathrm{W}}_{\text{acetanilide }}}{135}$

$\Rightarrow {\mathrm{W}}_{\text{acelanilide }}=\frac{1.86\times 135}{93} \mathrm{g}=2.70 \mathrm{g}$

But efficiency of reaction is $90\%$ only

$\therefore$ Mass of acetanilide produced$=2.70\times \frac{90}{100} \mathrm{g}$

$=2.43 \mathrm{g}$

$=243\times {10}^{-2} \mathrm{g}$

$\Rightarrow \mathrm{x}=243$