请教各位数学题

很快就要考试了，可数学部分还是没感觉，请教大家一下答案是如何得到的（我是从gmattutor.com 这个网站上看到的，说这个是数学的基本概念，可我都不知道如何正确翻译，急死了），求大家帮忙看一下，谢谢了 If three things are in a ratio of 5:9:11, what does the total number of things have to be a multiple of? 答案：25 (the sum of the numbers) If the number of things represented by the 5 doubles, can you represent the new ratio as 10:9:11? 答案：Yes, multiplication and division are okay If you add 6 things to the the number of things represented by the 9, can you represent the new ratio as 5:15:11? 答案：No, not unless you know the absolute number If the number of things represented by the 9 is reduced by one-third, can you represent the new ratio as 5:7:11? 答案： No, but you could represent it by 5:6:11