DRAM chip question

Question

A 32-bit wide main memory unit with a capacity of 1 GB is built using 256M × 4-bit DRAM chips. The number of rows of memory cells in the DRAM chip is 210. The time taken to perform one refresh operation is 100 nanoseconds. The refresh period is 3 milliseconds. The percentage (rounded to the closest integer) of the time available for performing the memory read/write operations in the main memory unit is______%

Comments

Pre-requisite to answer this question:

https://en.wikipedia.org/wiki/Memory_refresh#How_DRAM_refresh_works

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Refresh period = $100 ns* 2^{10}= 0.1024 ms$

% time for read/write = $(3 - 0.1024)*100/3 =96.58$%=$ 97 $%

Refer: https://gateoverflow.in/204097/gate2018-23

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https://www.youtube.com/watch?v=cjNORC_00_A

DRAM Starts from 21 minutes onwards 

hope this could help !!!