Difference between revisions of "2013 AMC 10A Problems/Problem 17"
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==Solution== | ==Solution== | ||
| − | The 365-day time period can be split up into 6 60-day time periods, because after 60 days, all three of them visit again. (Least common multiple of 3, 4, and 5) | + | The 365-day time period can be split up into <math>6</math> 60-day time periods, because, after <math>60</math> days, all three of them visit again. (Least common multiple of <math>3</math>, <math>4</math>, and <math>5</math>) |
| − | You can find how many times each pair of visitors can meet by finding the LCM of their visiting days and dividing 60 | + | You can find how many times each pair of visitors can meet by finding the LCM of their visiting days and dividing that number by 60 |
Remember to subtract 1, because you do not wish to count the 60th day, when all three visit. | Remember to subtract 1, because you do not wish to count the 60th day, when all three visit. | ||
| − | A and B visit <math>{60} | + | A and B visit <math>\frac{60}{3*4}-1 = 4</math> times. |
| − | B and C visit <math>{60} | + | B and C visit <math>\frac{60}{4*5}-1 = 2</math> times. |
| − | C and A visit <math>{60} | + | C and A visit <math>\frac{60}{3*5}-1 = 3</math> times. |
| − | This is a total of 9 visits per 60 day period. | + | This is a total of <math>9</math> visits per <math>60</math> day period. |
| − | Therefore, the total number of 2-person visits is 9*6 = 54 (B) | + | Therefore, the total number of 2-person visits is <math>9*6 = 54</math>, <math>\textbf{(B)}</math>. |
==See Also== | ==See Also== | ||
{{AMC10 box|year=2013|ab=A|num-b=16|num-a=18}} | {{AMC10 box|year=2013|ab=A|num-b=16|num-a=18}} | ||
Revision as of 21:46, 7 February 2013
Problem
Daphne is visited periodically by her three best friends: Alice, Beatrix, and Claire. Alice visits every third day, Beatrix visits every fourth day, and Claire visits every fifth day. All three friends visited Daphne yesterday. How many days of the next 365-day period will exactly two friends visit her?
Solution
The 365-day time period can be split up into 
60-day time periods, because, after 
days, all three of them visit again. (Least common multiple of 
, 
, and 
)
You can find how many times each pair of visitors can meet by finding the LCM of their visiting days and dividing that number by 60
Remember to subtract 1, because you do not wish to count the 60th day, when all three visit.
A and B visit 
times.
B and C visit 
times.
C and A visit 
times.
This is a total of 
visits per day period.
Therefore, the total number of 2-person visits is 
, 
.
See Also
| 2013 AMC 10A (Problems • Answer Key • Resources) | ||
| Preceded by Problem 16 |
Followed by Problem 18 | |
| 1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 | ||
| All AMC 10 Problems and Solutions | ||
