Hi there!
So here's what we know:
Alexander payed 5/6 of $1200
The word "of" is the same thing as a multiplying sign :
5/6 × 1200 = Price he payed
5 ÷ 6 = 0.83333333
0.83333333 × 1200 = 1000
So your answer is: Alexander payed $1000 for his computer.
There you go! I really hope this helped, if there's anything just let me know! :)