A processor executes all non-branch instructions in one cycle. This processor implements branch prediction, which incurs an additional penalty of 2 cycles if the prediction is correct and 4 cycles if the prediction is incorrect. 

a. If conditional branch instructions occupy 15% of the instruction stream, and the probability of an incorrect branch prediction is 20%, what is the average number of cycles per instruction? 

b. If the same processor is to run no less than 28% slower than a machine with a zero branch penalty when up to 20% of the instructions are conditional branches, what level of accuracy must the branch prediction achieve on average?