Solution

Since the image is formed on the screen, the image is real. A concave lens cannot form a real image. Therefore, the lens is convex.
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Object distance, u = -30 cm
Image distance, v = +60 cm
According to lens formula:
1/f = 1/v - 1/u
1/f = 1/60 - 1/(-30)
1/f = 1/60 + 1/30
1/f = 3/60
f = 20cm
Magnification of convex lens, m = v/u
h2/h1 = v/u
h2/3 = 60/(-30)
h2 = 60x 3/(-30)
h2 = -6cm
Thus the image of the candle flame is 6 cm high and inverted