Let the number of passengers be 1u

1u – 1/3u = 2/3u
60 – 220 + 60 = -100
  2/3u – 100 = 5/9u 
2/3u – 5/9u = 100
             1/9u = 100
             5/9u = 100 x 5 = 500 as (a)’s answer

            9/9u = 100 x 9 = 900 as (b)’s answer