Remember the two data sufficiency statements never contradict each other. So if they are sufficient, they will lead you to a same unique solution.

Total price of n shares of a stock= $12,000
total shares = n
price of 1 share of the stock = $12,000/n

rephrasing the question: what is the value of n?

statement 1: SUFFICIENT
new total price = $12,300
new price per stock = 12,300/n = (12000/n)+1
=>12300/n - 12000/n = 1
=> n=300

Think this in a different way

If price of each share of a stock is increased by $1, then how many shares should I have so that the total amount increased would be $300? Ans is 300 shares.

statement 2: SUFFICIENT

If I have 300 shares(the unique answer that we got from statement 1) and the price of each share of a stock is decreased by $2, then by what amount the total price would of the shares would be decreased? Ans is $600.
Check whether 5% of 12000 is $600. Ans yes.
Hence both the statements are sufficient.

Thank you very much for the great explanation, niketdoshi123. I get it now.