Explanation

To find the power of the combination of lenses, we can use the formula:

P_total = P1 + P2

Where:

- P_total is the total power of the combination

- P1 is the power of the convex lens

- P2 is the power of the concave lens

The power of a lens is given by the formula:

P = 1 / f

Where:

- P is the power of the lens

- f is the focal length of the lens

Given that the focal length of the convex lens is 40 cm and the focal length of the concave lens is -25 cm (negative because it is concave), we can calculate their respective powers:

P1 = 1 / 40 cm = 0.025 dioptres (approximately)

P2 = 1 / -25 cm = -0.04 dioptres (approximately)

Now, we can calculate the total power of the combination:

P_total = P1 + P2

= 0.025 dioptres + (-0.04 dioptres)

= -0.015 dioptres / -1.5  (approximately)

(0.015 = 1.5= 0.015 * 100= 1.5)