Use the following ray diagram, Fig. to calculate focal length of lens `L_2`.
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Explanation

For lens `L_1, f = 15 cm`,
`u = -40 cm`,
`v = ?`
`(1)/(v)=(1)/(f)+(1)/(u)=(1)/(15)-(1)/(40)=(5)/(120), v = 24 cm`
The image would have been at `I'`,
where `L_1 I' = 24 cm`.
`I'` serves as object for `L_2`
`u = +(24 - 14) = 10 cm, v = 30 cm, f' = ?`
`(1)/(f')=(1)/(v)-(1)/(u)=(1)/(30)-(1)/(10)=-(1)/(15) f = -15 cm`.