$1.$ Assume that in a particular system A with no cache, a memory reference takes $80$ ns$.$ Further assume that another system B has both L$1($primary cache$)$ and L$2($secondary cache$).$ Let Ah and Bh be the hit ratios of L$1$ and L$2$ respectively. Let Ma $=10$ ns$,$ Mb $=20$ ns be the hit times of L$1$ and L$2$ respectively. M $=60$ ns is the time taken to access memory in case of misses in both L$1$ and L$2,$ in system B$.$ If Ah $=$ Bh $=$ h$,$ what is the minimum hit ratio $($h$)$ that must be achieved in system B in order for its performance to equal the performance of system A$?$