这个小五数学题应该怎么讲解？

狮陀岭 · 发表于 27-8-2013 23:15:34|来自：新加坡

本帖最后由狮陀岭于 27-8-2013 23:21 编辑

The ratio of the number of children to the number of adults at a partywas 2:5. After 35 children and 35 adults joined the party, the ratio of the
number of children to the adults became 3:4. How many children were there at first?

我会设X列方程做，当然也可以猜着做，猜到5就做出来了。但这都不适合小五学生来理解。
请问有没有适合小五学生的解法？
谢谢

davidbin · 发表于 28-8-2013 08:31:36|来自：新加坡

Constant difference
2:5 ( 5-2 =3) X1 ------------->2:5
3:4 (4-3 =1) X3 -------------> 9:12
9u-2u = 7u = 35
1u= 5
2u = 10
there were 10 children at first

fangchaoxx88 · 发表于 28-8-2013 09:07:28|来自：新加坡

楼上正解，此类题，无非就是总数不变，差值不变。最难的就是什么都变的，就用交叉相乘。其实和解方程的概念是一样的，只是更加适合小学生去理解。

TomorrowWalk · 发表于 28-8-2013 10:10:49|来自：新加坡

2/5 ==> 3/4; ( the ratio changed from 2/5 to 3/4 after additional 35 each children & adult added )
8/20 ==> 15 / 20;
8*u + 35 = 15 * ( u + 35/20 ); ( 8u is the original number of children )
u = 1.25 ;
8u = 10

同“fangchaoxx88”说的，基本意思是一样的，其它guess、list 之类的方法也可以玩玩，只要孩子能有兴趣接受就成。