Let a litres be the amount of the stronger alcohol solution, then 280-a litres of the other solution are mixed with it to produce 280L altogether. The amounts of pure alcohol mixed are represented by:
0.745a+0.545(280-a)=0.595×280=166.6, meaning that the final solution contains 166.6L of pure alcohol.
So, 0.745a+152.6-0.545a=166.6, 0.2a=166.6-152.6=14, a=14/0.2=70L.
So initially there was 70L of 74.5% alcohol solution.