prep的一道题

mvmv90

Of the 75 houses in a certain community, 48 have a patio.  How many of the houses in the community have a swimming pool?



(1) 38 of the houses in the community have a patio but do not have a swimming pool.



(2) The number of houses in the community that have a patio and a swimming pool is equal to the number of houses in the community that have neither a swimming pool nor a patio.

此题我怎么都想不通为什么只用第二个条件就可以算出答案

                                      patio   no-patio

                       pool           a            b

                     no-pool        c            a  

                                      48           27(75-48)

以上就是单靠第二个条件所能得出的信息，how could it get the answer directly...???

tonger

LZ画个图吧，就是那种两个圈圈相交的图...
然后你就会惊讶地发现，只有游泳池而没有露台的人数，加上两个都有的人数，就等于拥有游泳池的人数,,,而两个都有的，等于两个都没有的...而两个都没有的加上只有游泳池的，就是75-48...

mvmv90

感谢LS！顺便想问一下为什么我那种方法算不出结果呢？我一直以为那种方法万能的...

thx again

jiuwang

just use your information:

a + b + a + c = 75 (total house)
a + c = 48 (house with patio)
question: a + b = ? (house with pool)

mvmv90

right...got u...thx u, both of u, so much indeed