a convex lens of power 5D is combined with a concave lens of power 4D,the focal lenght of the combination is?

Focal length of convex lens
f=1/P=1/5=0.2 m=20 cm
Similarly,
Focal length of concave lens
=- 0.25 m=-25 cm
Combined focal length of lens combination is given by
= f+ f'
=20-25
=-5 cm
Negative sign shows that combination of lens is concave lens.