Explanation

(a) When two lenses are placed coaxially at a distance $d$ from each other, then equivalent focal length (F) is given by
$\frac{1}{F} = \frac{1}{f₁} + \frac{1}{f₂} - \frac{d}{f₁f₂}$
When the lenses are put together,
$d = 0$
Hence, the focal length of the combination will decrease.