Width of 20 maxima of double slit = width of central maxima of single slit
$\begin{aligned}
& \frac{20 \lambda \mathrm{D}}{\mathrm{d}}=\frac{2 \lambda \mathrm{D}}{\mathrm{a}} \
& \frac{10}{\mathrm{d}}=\frac{1}{\mathrm{a}} \
& \mathrm{a}=\frac{\mathrm{d}}{10}=\frac{1.5 \times 10^{-1}}{10} \mathrm{cm}=15 \times 10^{-3} \mathrm{cm}
\end{aligned}$
Value of x is 15
Answer is 15