As we know cpu time = [Instruction Count ( IC) * CPI * Clock time] ,
for M/c A cpu time is = IC*4.0 * 50 = 200 IC ns ,
for M/c B cpu time = 162.5 IC ns ,
as ISA is same for both then IC is equal for both,
M/c A takes much time ( longer cpu time ) to perform same program than M/c B , hence M/c B is faster.