The prism is partially filled with 150 cubic cm of sand what fraction of the prism is filled by the sand

Since 150 can't go into 250 twice as a whole, you know that the percentage is over 50% . To find the percentage (I think, bad memory) You need to find the difference or increase between the two. 250 / 150 = 100 Then divide the increase or difference by the original number (As in 150) then multiply that by 100. The percentage would be 66% 

Hope this helps, though would ask someone again just to be sure