When in contact, the equivalent power of two thin lenses is, P1+P2=10.... (i) When the lenses are separated by 0.25m, P1+P2−0.25P1P2=6 ∴0.25P1P2=4 ∴P1P2=16 P1−P2=√(P1+P2)2+4P1P2 ∴P1−P2=√36=6....(ii) Adding equations (i) and (ii), we get, P1=8D ∴P2=10−8=2D