分分求教一道ratio题

jihua1026

妞在学校做了一道ratio题，答案是81,可我得到的答案是63，到底哪个对，为啥呀
题目如下：
The ratio A to B is3:4,If 18 people join B,the ratio is now 1:2, How many people are there in total?

qkd

Original A & B with total 7 units (i.e. 3+4)
After 18 people join B ... A & B total with 9 unit (1:2 = 3:6 ...3+6 = 9)
9 units - 7 units = 18 people  ... so 1 unit = 9 people (18 people/(9-7))

Total (Original) = 9 people x 7units = 63 people
Total (After 18 people joined) = 9 people x 9units = 81 people.

CHECK 81-63 people = 18 people (i.e. correct)

jihua1026

qkd 发表于 2015-9-25 10:34
Original A & B with total 7 units (i.e. 3+4)
After 18 people join B ... A & B total with 9 unit (1:2 ...

忽略了新加入的18人啦  

frizzyhair2015

xiaobaoluo

是的，应该是81.

aileenzhang

63是原来AB的总数，81 算上了后来的18个人

A/B= 3/4 ； A/（b+18)= 1/2  => A =27 ,B=36

fhjp01

参考我给你的另外一道题的图解法来解释。