Answer

Regional Level Solution
.......

12

1. Let d be the common difference in our arithmetic sequence.
Then x=6d while y=6+d, making x+y
= (6d)+(6+d) = 12. Notice we didn't need to
use the fact that the first term equaled 1. (Alternately, one could deduce
that d=2.5 since the difference between the first and third terms
was 5, then computed x=3.5 and y=8.5 so that x+y=12.)

.
2

2. The region in which the two squares overlap with the rectangle is a
right triangle, which we shall call T. Because the squares each
have area 1, the area of T is also 1. However, upon closer inspection
we see that the base and height of the rectangle are the same as the (slanted)
base and height of triangle T. Since area(T)=bh/2=1,
we find bh=2, so the area of the rectangle is 2. Alternate
solution: can the reader figure out how to dissect the two squares into
pieces that can be reassembled into the rectangle, thereby showing that
they must have the same area?

.
12

3. Each of the numbers from 1 to 6 can only have two
possible neighbors to avoid sums divisible by 2 or 3. For
example, 5 may neighbor only 2 or 6. If we write the numbers
1, 4, 3, 2, 5, and 6 in that order in a ring then each
number will be next to its two possible neighbors. (Verify.)
Therefore, to order the six numbers successfully we need
only start at any point along the ring (6 choices) and list
the numbers as they appear around the ring in either
direction (2 choices) for a total of 12 possible
orderings.

.
60

4. Since 30 must divide 72N, there must be factors of 2, 3, and
5 in 72N. The 2 and 3 are already present in 72, so N
must be divisible by 5. In the same manner, since 30N is a multiple
of 72, N must be a multiple of 12. The smallest positive integer
divisible by both 5 and 12 is 60. As a check, we note that 60 divides
evenly into (30)(72), so N=60 satisfies all the requirements.

.
70

5. In the time it takes the older sister to reach the top,
the up escalator has carried her forward 30 steps, because
the total length of the escalator is 40 steps and she took
10 steps herself. Therefore in this same time the down
escalator pushes the younger sister back 30 steps. To make
up this setback and cover the original 40 steps separating
her from the top the younger sister must make a total of
70 steps.

.
1/12

6. If I choose a shot that I will make with probability p (where
p is between 0 and 1/3), then Michael Jordan will make the same
shot with probability 3p. Hence the probability that I make a
shot that Jordan subsequently misses is p(13p). The
graph of this function is a parabola which equals zero when p=0
and p=1/3. By symmetry the vertex (maximum) is midway between
the two, at p=1/6. Hence the best chance I have of winning the
game is 1/12.

.
29pi

7. Notice that a right triangle with legs whose squares are 17 and 99
has the same hypotenuse as a right triangle with legs whose squares are
19 and 97, namely the square root of 116 in both cases. Now place these
two right triangles next to one another (not overlapping) so that their
hypotenuses coincide. The circle with the common hypotenuse as diameter
will neatly circumscribe the resulting quadrilateral since any angle inscribed
in a semicircle must be a right angle, and vice versa. To summarize: we
have created a quadrilateral with the appropriate side lengths inscribed
in a circle with diameter equal to the square root of 116, and hence with
area 29pi.


Reg Level Test
 Return to samples page
