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Transactions involved in purchasing : 5*4 = 20

Transactions involved in selling : 3*2= 6

therefore total transaction needed is 6 to increase the computer by (20-6) 14

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?

Note the question does not state that a transaction is "buy 5, sell 3". If it were, the answer would be 7.

The question states that buying 5 is a transaction and selling 3 is a transaction. "...each of these purchasing or selling is regard as a transaction."

A selling transaction is -3
A purchasing transaction is 5

What combination of multiples (number of transactions) of 5 and -3 make 14? The first thought is that (at least meaning the lowest number) the multiple of 5 has to either be 14 (which is not possible since 14 isn't a multiple of 5) or greater than 14 since the -3 multiples (transactions) subtract from it.

Multiples of 5: 5 (1 transaction, 5x1=5), 10 (2 transactions, 5x2=10), 15, 20, 25, 30

5x4=20. That's the smallest multiple of 5, the smallest number of transactions that add 5. That's 4 transactions. Is there a multiple of -3 that when added to that makes 14?

Yes! -3x2=-6 (2 transactions.)

4+2=6

Yep, i overlooked the "or" and assumed that the buying and simultaneous selling amounted to one transaction!