Responding to a PM
PalakDiwanji wrote:
Hi, sorry but could you please explain what I'm doing wrong here?
If 20 students take math and 10 take history, and 7 take both:
Only math = 20 -7 = 13
Only history = 10 - 7 = 3
So isn't the answer that 16 take either math or history?
There are two ways one can denote A or B :
(1) A + B - Both AB
(2)Only A + Only B + Both AB
So you have left out the students who are taking both. They need to be included.
The first point can be broken up as :
(1) A + B - Both AB : Only A + Both AB + only B + Both AB - Both AB -> Only A +Only B + Both AB , this is your second formula.
So Both AB needs to be accounted for only once, in the first case we counted them twice hence we need to subtract one of them. Hence we have the formula: A + B - Both AB
For A or B you cannot leave out the ones who are taking both, you need to count them once. That is the problem with your solution.
Hope it's clear.
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