The correct answer is (a) 30 cm to the right of the third lens
To elaborate: Image by the first lens,
(frac{1}{V_1}-frac{1}{U_1}=frac{1}{f_1} )
(frac{1}{V_1}-frac{1}{-30}=frac{1}{10})
v1 = 15 cm
For the second lens,
(frac{1}{V_2}frac{-1}{-10}=frac{1}{10})
V2 = ∞
For the third,
(frac{1}{V_3}frac{-1}{infty}=frac{1}{30})
V3 = 30 cm.