
Word problem
A dart board is designed to have two scoring areas, as shown. If an unlimited number of darts is allowed, what is the largest finite score that cannot be attained?
I couldn't draw the two circles so I'll explain. If the dart landed on the outer circle if would count for 7 points and the inner circle would count for 15.
I see that I will be able to get all the multiples of 7
7 14 21 28 35 42
and then if i get a 15 and then all 7 i can get all the
15 22 29 36 43
and I get 7 and all 15..
7 22 37 52...
and I can't see how to get the largest finite score that can't be attained.
I tried other combinations also. Maybe i am doing it too randomly.
Could you please explain the logic of this problem step by step.
Thanks.
Vicky.

If you put x darts into the 15 point area and y darts into the 7 point area, you get 15x+ 7y points. So the mathematics question you are asking is "what is the largest number that cannot[ be written 15x+ 7y for nonnegative integers x and y."
7 divides into 15 twice with remainder 1 so 15(1)+ 7(2)= 1. If A is any number then 15(A)+ 7(2A)= A. x= A, y= 2A will give any number A. Of course, 2A is not a nonnegative number. In fact, x= A 7k and y= 2A+ 15k is also a solution for any integer k: 15(A 7k)+ 7(2A+ 15k)= 15A+ 7(15)k 14A+ 7(15)k= A since the "7(15)k" terms cancel out.
So we must have and . Those are the same as and :
Well, obviously, that is always possible if there is an integer between and . In particular it is certainly possible if . That is, it is possible for any A> 105.
But that doesn't mean it isn't possible for any number less than or equal to 105. But at least we have only a finite number of cases to check:
If A= 105, and so k= 14 or 15 will work. If k=14, x= 105 7(14)= 105 98= 7 and y= 210+ 15(14)= 210+ 210= 0. You can get 105 points by putting 7 darts in the 15 point circle and none in the 7 point circle. If k= 15, x= 105 7(15)= 0 while y= 201+ 15(15)= 15. You can get 105 points by putting 0 darts in the 15 point circle and 15 points in the 7 point circle.
If A= 104, [tex]= 13 and 13/15 while = 14 and 6/7. k= 14 is between them. x= 104 7(14)= 6 and now y= 2(104)+ 14(15)= 208+ 210= 2. You can get 104 points by putting 6 darts in the 15 point circle and 2 darts in the 7 point circle.
If A= 103, = 13 and 11/15 while = 14 and 6/7. k= 14 is still between those two numbers. x= 103 7(14)= 5 and y= 2(103)+ 14(15)= 206+ 210= 4. You can get 103 points by putting 5 darts in the 15 point circle and 4 darts in the 7 point circle.
If A= 102, = 13 and 3/5 while = 14 and 4/7. k= 14 is still between those two numbers. x= 102 7(14)= 4 and y= 2(102)+ 14(15)= 204+ 210= 6. You can get 102 points by putting 4 darts in the 15 circle and 6 darts in the 7 point circle.
If A= 101, = 13 and 7/15 while = 14 and 3/7. k= 14 is still between those two integers. x= 101 7(14)= 3 and y= 2(101)+ 14(15)= 202+ 210= 8. You can get 101 points by putting 3 darts in the 15 circle and 6 darts in the 7 circle.
If A= 100, = 13 and 1/3 while = 14 and 2/7. k= 14 is still between those two integers. x= 100 7(14)= 2 and y= 2(100)+ 14(15)= 200+ 210= 10. You can get 100 points by putting 2 darts in the 15 point circle and 10 darts in the 7 point circle.
If A= 99, = 13 and 1/5 while = 14 and 1/7. k= 14 is still between those two integers. x= 99 7(14)= 1 and y= 2(99)+ 14(15)= 198+ 210= 12. You can get 99 points by putting 1 dart in the 15 point circle and 12 darts in the 7 point circle.
If A= 98, = 13 and 1/15 while = 14. k= 14 again. x= 98 7(14)= 0 and y= 2(98)+ 14(15)= 196+ 210= 14. You can get 98 points by putting 0 darts in the 15 point circle and 14 darts in the 7 point circle.
If A= 97, = 12 and 14/15 while = 13 and 6/7. Now k= 13 will work.
If A= 96, = 12 and 4/5 while = 13 and 5/7. Again, k= 13 will work.
If A= 95, = 12 and 2/3 while = 13 and 4/7. k= 13 will work.
If A= 94, = 12 and 8/15 while = 13 and 3/7. k= 13 will work.
If A= 93, = 12 and 2/15 while = 13 and 2/7. k= 13 will work.
If A= 92, = 12 and 4/15 while = 13 and 1/7. k= 13 will work.
If A= 91, = 12 and 2/15 while = 13. k= 13 will work.
If A= 90, while = 12 and 6/7. k= 12 will work.
If A= 89, = 11 and 13/15 while = 12 and 5/7. k= 12 will work.
If A= 88, = 11 and 2/3 while = 12 and 4/7. k= 12 will work.
If A= 87, = 11 and 3/5 while = 12 and 3/7. k= 12 will work.
If A= 86, = 11 and 7/15 while = 12 and 2/7. k= 12 will work.
If A= 85, = 11 and 1/3 while = 12 and 1/7. k= 12 will work.
If A= 84, = 11 and 1/5 while = 12. k= 12 will work.
If A= 83, = 11 and 1/15 while = 11 and 6/7.
Finally! There is no integer between 11 and 1/15 and 11 and 6/7 so there are no x and y that will make 15x+ 7y= 83.
Since every number large than 83 can be written that way, 83 is the largest number that cannot be written 15x+ 7y and so is the largest number of points that cannot be achieved by throwing darts at 15 point and 7 point circles.

Thank you so much for your help. But I have to admit I don't fully understand the solution yet. I think I will be able to if I study it a little more with some guessing and checking.
You also saved me a lot of time cause I was starting to write down all the possible numbers and it was getting very messy.
Vicky.