ETF Round trip commission

Hi there! I’ve got a question about the Round-trip Commission in calculating the ETF’s total cost.

Here is the question: the bid-ask spread is 0.15%, the ETF commission is 0.1%, and the annual management fee is 0.08%. It is asking for a 3 months average annual total cost.

the Answer is 20.1%+0.15%+0.08%(3/12)=0.37%

Can anyone tell me why does it have to multiply a 2 for the commission, please?

much thanks,

purchase and sale of an ETF.

Thank you!
Good luck in the exam!