beater wrote:A computer retailer bought 5 computers each time and sold 3 computers each time, and each of these purchasing or selling is regard as a transaction. Last month, if the computers in stocks increased by 14, at least how many transactions were performed during last month?
A. 3
B. 5
C. 6
D. 7
E. 8

First of all, this is a poorly worded question and clearly not an official GMAT question. I understand why you want help with it.

Second, your name is "beater", which is hilarious so I am some-what amused.

The difference between both actions of each transaction is +2.
Gain 5, lose 3.... 5 - 3 = 2

If gain 14 comps in a month, then you've had 7 transactions (14/2 = 7)