View source for 2017 AMC 8 Problems/Problem 24

==Problem==

Mrs. Sanders has three grandchildren, who call her regularly. One calls her every three days, one calls her every four days, and one calls her every five days. All three called her on December 31, 2016. On how many days during the next year did she not receive a phone call from any of her grandchildren?

<math>\textbf{(A) }78\qquad\textbf{(B) }80\qquad\textbf{(C) }144\qquad\textbf{(D) }146\qquad\textbf{(E) }152</math>

==Solution 1==

For this problem, we use the Principle of Inclusion and Exclusion(PIE). Notice every <math>3</math> day a day would be a multiple of <math>3</math>, every <math>4</math> day a day is a multiple of 4, and every 5 days it's a multiple of <math> 144</math> days without calls. Note that in the last five days of the year, days <math>361</math> and <math>362</math> also do not have any calls, as they are not multiples of <math>3</math>, <math>4</math>, or <math>5</math>. Thus our answer is <math>144+2 = \boxed{\textbf{(D)}\ 146}</math>.

<cmath></cmath>

==Solution 2==
Since there are 365 days in the year, <math>365\cdot \frac45 \cdot \frac 34 \cdot \frac23=\boxed{146}</math> days without calls.

~Gr8

==Solution 3==
We use Principle of Inclusion and Exclusion. There are <math>365</math> days in the year, and we subtract the days that she gets at least <math>1</math> phone call, which is <cmath> \left \lfloor \frac{365}{3} \right \rfloor +  \left \lfloor \frac{365}{4} \right \rfloor +  \left \lfloor \frac{365}{5} \right \rfloor</cmath>

To this result we add the number of days where she gets at least <math>2</math> phone calls in a day because we double subtracted these days. This number is <cmath>\left \lfloor \frac{365}{12} \right \rfloor +  \left \lfloor \frac{365}{15} \right \rfloor +  \left \lfloor \frac{365}{20} \right \rfloor</cmath>

We now subtract the number of days where she gets three phone calls, which is <math>\left \lfloor \frac{365}{60} \right \rfloor</math>. Therefore, our answer is 

<math>365 - \left( \left \lfloor \frac{365}{3} \right \rfloor +  \left \lfloor \frac{365}{4} \right \rfloor +  \left \lfloor \frac{365}{5} \right \rfloor \right) +  \left( \left \lfloor \frac{365}{12} \right \rfloor +  \left \lfloor \frac{365}{15} \right \rfloor +  \left \lfloor \frac{365}{20} \right \rfloor \right) - \left \lfloor \frac{365}{60} \right \rfloor</math> 

<math> = 365 - 285+72 - 6 = \boxed{\textbf{(D) } 146}</math>.


==Video Solution==
*https://youtu.be/a3rGDEmrxC0 - Happytwin

*https://youtu.be/Zhsb5lv6jCI?t=2797

==See Also==
{{AMC8 box|year=2017|num-b=23|num-a=25}}

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