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1/v + 1/u = 1/f

so v = distance of object form mirror = infinity(i)
   u = distance of image from mirror = 7.5
 f = focal length of mirror = f
1/i + 1/7.5 = 1/f 
0 + 1/7.5 = 1/f
f = 7.5 cm ANSWER

U the dist of the object is assumed to be zero.     v the dist of the image=7.5cm         f the focal length=?  theradius of curvature=2f=?     using the equn 1/u +1/v=1/f        0+1/7.5=1/f or f the focal length=7.5cm  rthe radius of curvature=2f =2 x7.5 15cm