(i) If final image is at infinite, for this image by one lens should be formed on focus of other.
Image by `L_1`:
`(1)/(v)-(1)/(-x)=(1)/(-20)implies(1)/(v)=-(1)/(20)-(1)/(x)=(-(20+x))/(20x)`
`v=-(20x)/((20+x))`
`(20x)/(20+x)+15=30implies(20x)/(20+x)=15`
`4x=60+3x`
`x=60cm`
(ii) Image by `L_1`:
`(1)/(35)-(1)/(-x)=(1)/(30)implies(1)/(x)=(1)/(30)-(1)/(35)=(7-6)/(210)`
`x=210cm`