$\text{Read Hit}:$ The main memory is not involved when there is a cache hit in a Read operation
$\text{Read Miss}:$ In this situation There are two cases :
- The block of words which containing the requested word copied from the main memory to cache and when block loaded into cache then requested word sent to the processor.
- Requested word may be sent to the processor as soon as it reads from the main memory.
In this question time given when cache miss so no need to think about cases.
The average read access time :
Assuming there are $100$ read requests and $80\%$ of it results into cache hit and remaining cache miss so,
$t_{avg} =\frac{ \frac{80}{100}\times100 \times 5ns + \frac{20}{100}\times100 \times 50ns}{100}= 14ns $
so $14ns$ should be the right answer
Reference : $\text{Hamacher Sixth Edition : P.no 290-91}$