for the combinations of lenses we use this formula i/F=1/f1+1/f2

given that , power of convex lens =+2.5D

we know that ,power =100/focal length(in cm)

focal length =100/2.5(in cm)

f1=40 cm

and given that ,focal length of concave lens is 25 cm

f2=-25 cm (because focal length is negative in concave lens

therefore 1/F=1/f1+1/f2

1/F=1/40-1/25

1/F=-3/200

therefore,F=-200/3 cm

power=100/(-200/3)=-3/2=-1.5D