In such questions, I personally always find it easier to map the data in a tabular form.
(I don't know if there is a name for this method)
Anywho, first and foremost, using the data given in the question, make a table as given below:
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I have marked the unknown ie. computers >1TB and >3.2 G Hz as
xI have marked the other unknown ie. Total no. of computers in the store as
TNow using the table, we can form the following equations:
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Now lets take the bottommost horizontal row, (Even the rightmost vertical column can be considered

)
We know 55 + 60 - x = T
115-x = T
<------------- (1)We know that T has to be greater than 100.
Plugging in the values into (1), we get
I) x = 10, T = 105 ----> Hence, possible.
II) x= 15, T = 100 ----> Not possible as T>100.
III) x = 20, T = 95 -----> Again, not possible.
Hence only (I) is possible.
Option A is our answer.