\(\frac{1}{F} = \frac{1}{{{f_1}}} + \frac{1}{{{f_2}}} - \frac{d}{{{f_1}{f_2}}}\)
==> \(\frac{1}{\infty } = \frac{1}{{ + 30}} + \frac{1}{{ - 10}} - \frac{d}{{\left( {30} \right)\,\left( { - 10} \right)}}\)
==> \(d = 20 cm\)
(શૂન્યવકાશમા પ્રકાશનો વેગ $=3 \times 10^{8} \,{m} / {s}$ અને $\left.\cos 30^{\circ}=\frac{\sqrt{3}}{2}\right)$