A computer system was seen to have an improvement in the average memory access time from $7\mathrm{.}6ns$ to $4\mathrm{.}9ns$ by improving the hit rate of the cache. Assuming the main memory has an average access time of $10ns$ and the cache has an average access time of $4ns,$ what improvement in cache hit rate was obtained?

$0\mathrm{.}45$

$0\mathrm{.}4$

$0\mathrm{.}85$

$0\mathrm{.}2$