"Every 3rd instruction needs a 1– cycle stall before the X-stage"
therefore for 100 instructions , clock cycles will be = 100/3 = 33 clock cycles stalls
And the time taken for each cycle is = max( 9,3,7,9,2) + 1ns of overhead = 9+1 = 10ns
therefore for 33 clock cycles it take = 33 * 10 = 330 ns
And the time taken by CPU to execute 100 instructions is
= 1(1st instruction) * 5(CPI) * 10(each clock cycle time) + 99(remaining 99 instruction) * 1 ( CPI ) * 10(each clock cycle time)
= 1040 ns
Total time taken by CPU = 330 + 1040 = 1370 ns Ans.