Memory request for read $=0.75$
Memory request for write $=0.25$
Now, it is given for read request $90$% is hit and write through protocol is used.
In write through protocol, cache and main memory updated simultaneously.
- So, for Read time required $=0.75(0.9*45+0.1*750)=86.625ns.$
- for Write time required (as hit ratio of write is $1$)$=0.25*750=187.5$
Total time required $=86.625+187.5=274.125ns.$