total memory references: 2000
1st check in L1..miss=320 hit=1680
now for 320 misses in L1 check L2..miss=150 hit=170
avg mem acc time=
[1680×1 + 170×(10+1) + 150×(300+10+1) ] ÷ 2000
=25.1
another way to approach is-->
for every 2000 mem acc L1 is accessed
so 1 clk CYCLE..................(a)
for every 320 misses in L1 , L2 is accessed
so 320×10/2000 clk CYCLEs............(b)
for 150 misses in L2 , penalty is 300 clk CYCLEs
so 150×300/2000 clk CYCLEs............(c)
adding a+b+c :
1+3200/200+45000/2000 = 25.1
ANSWER: C)