Two thin lenses are of same focal lengths f , but one is convex and the other one is concave. When they are placed in contact with each other, the equivalent focal length of the combination will be:

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The formula to calculate the effective focal length of the combination can be written as
$\frac{1}{f_{eff}} = \frac{1}{f_{convex}} + \frac{1}{f_{concave}} . . . (1)$
Substitute $f$ for $f_{convex}$ and $- f$ for $f_{concave}$ into equation (1) and solve to obtain the effective focal length of the combination.
$\begin{array}{rcl} \frac{1}{f_{eff}} & = & \frac{1}{f} - \frac{1}{f} \\ = & 0 \\ \Rightarrow & f_{eff} = \infty \end{array}$

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