From the lensmaker's equation, we have a symmetrical lens with R1 = -R2, and because it is a convex lens, R1>0. Then:
1/(15 cm)= (n-1)*(1/R1-1/R2)
1/(15 cm)= (n-1)*(2/R1)
1/(30cm) = (n-1)/R1
The new lenses produced by cutting the original lens in half (and we assume the cut is a flat surface) will have focal lengths:
1/f = (n-1)*(1/R1 + 1/infinity)
1/f = (n-1)*(1/R1 + 0)
1/f = (n-1)/R1
But we already showed that (n-1)/R1 = 1/(30 cm), so the focal length of each half of the lens is 30 cm.
