CPU Execution Time / Performance
Since the clock rate is fixed, the number of instructions needed for the "new" P2 is 8/10 * 30 x 10^9, or 24 x 10^9. That is, if you reduce the number of instructions to 80% of the original, the execution time drops to 80% as well.

Note that the question made no mention of cycles per instruction. It just asked how many instructions are needed to produce the desired execution time.