A symmetric double convex lens is cut in two equal parts by a plane perpendicular to the principal axis. If the power of the original lens was 4 D, the power of a divided lens will be

Leaving ahead all mathematical formulations, we can answer this by simple intuition. Even after cutting the lens, if we place them together and provide rays of light, commonly we know, it will generate the same image point. So, still, the combined power is 4D. But, after cutting the they became regular and exactly equal halves. And also Power is a scalar property, hence power of each lens, must be 2D.