L1 miss 65% means 1-H = 65 % and H = 35%
formula is :
TAvg writing Time = H * T mem update time + ( 1 - H ) * ( T mem update time )
= 0.35 * 50 + 0.65 * 50
= 17.5 + 32.5
So average writing time for system is 50 ns .
In write through both cache and MMU update same time .
Write Through uses Write No-Allocate policy, in Write No-Allocate Missed main memory block is updated while residing in the Main memory itself and NOT brought to the cache , that's why only MMU update time is consider here .