math OG Data Sufficiency 49!!!

i am stuck with a problem here:  I appreciate anyone's help with the problem! Thanks a lot!!!

OG Data Sufficiency  49:

How many people are directors of both company K and company R?

1)There wre 17 directors present at a joint meeting of the directors of company K and company R and no directors were absent.

2) Company K has 12 directors and company R has 8 directors.

Correct Answer: C

Both Statement together are sufficient.

It is said in the answer that there are 20 directorship in all, so 20-17=3 are joint directors.

What is actually "joint director"? Does that mean people who work in both companies?

I would appreciate if anyone could help me! Thanks a lot!!!

??

从数学上理解反正就知道是交集就可以了

楼主在北美的话别忘了时差问题，这会国内同学在睡觉的，等回帖别心急么

Yep,"joint directors"means these directors work for both company K and R. So, the answer is

12+8-17=3

thanks a lot!!

sorry.. i did forget the time...

That's fine, im in London

这是一道小学鸡兔同笼的体型，看到后根本不用算，只要总位置数大于总人数，那么一定有解，而且总人数和总位置数必须都知道。

这道题如果变一下，A变成总共有21个人参加，B不变，则恒无解，E