Zaheen uses a convex lens to make sunlight burn a hole in a piece of card. Zaheen changes the lens in the holder for another lens with twice the power. What will she have to do to focus the sunlight on the card?

(N/A) Power is related to the focal length as $P = \frac{1}{f}$.
An increase in power means a decrease in focal length.
Since the sunlight comes from infinity,the image is formed at the focal point of the lens.
If the power is doubled $(P' = 2P)$,the new focal length becomes half of the original focal length $(f' = \frac{f}{2})$.
Therefore,to focus the sunlight on the card,she will have to move the lens closer to the card,specifically at a distance equal to half of the previous focal length.