When you hold a convex (converging) lens in the sunlight over a piece of paper the paper ignites. (a) What is the likely focal length of the lens if

When you hold a convex (converging) lens in the sunlight over a piece of paper the paper ignites. (a) What is the likely focal length of the lens if it was held 15 cm above the paper?(b.) Would the paper ignite if you held the lens 10cm away?