Bunuel
In a class comprising boys and girls, there were 45 hand shakes amongst the girls and 105 hand shakes amongst the boys. How many hand shakes took place between a boy and a girl, if each member of the class shook hands exactly once with every other student in the class?
A. 15
B. 24
C. 25
D. 150
E. 300
Breaking Down the Info:First, we need to find the number of handshakes made with n people.
Each person should handshake each other person (n - 1 people). Repeat this for all n people so that would be \(n*(n - 1)\) handshakes in total. The flaw with this method is that it double counts A shaking with B then B shaking with A, so we must divide that by 2 to get the real number of handshakes, which is \(\frac{n*(n - 1)}{2}\).
Now we may set \(\frac{n*(n - 1)}{2} = 45 \text{ or } 105\) to find \(n = 10\) or \(n = 15\) (10*9= 90 and 15*14 = 210).
Thus there are 25 students in the class, and plugging this in the formula gives \(25*12 = 300\).
Answer: E