Category Archives: brain puzzler

October 2018 Brain Puzzler Solution

Q: A Math quiz has $3$ multiple-choice questions, each with $3$ choices. A student passes the quiz with $2$ or more correct questions. If the student randomly picks their answers, what is the probability of passing the quiz?

A: 7/27  = 1/27 + 6/27
Scenario 1: The student gets all 3 correct (1/3) x (1/3) x (1/3) = 1/27
Scenario 2: The student gets 2 correct.  1/3 x 1/3 x 2/3 but this can happen 3 different ways as the student could get the 1st one incorrect or the 2nd or the 3rd.
so (1/3)x (1/3) x (2/3) x 3 = 6/27
1/27 + 6/27 = 7/27.

August 2018 Brain Puzzler Solution

Q: If you only have 4s and 5s to make other numbers (such as 4 + 4 + 5 = 13 and 5 + 5 + 5 + 5 + 5 = 25),  how many numbers from 1 to 1000 have this property ?

A: 994
Which #s cannot be made?  There are 6 numbers that cannot be made:1 2 3 6 7  or 11.
That leaves 994 numbers.
All others can be made from 4s and 5s as 8 is 2 4s and 9 is 4 and 5 and 10 is 5 and 5.
After 12 which is 3 x 4, all other numbers can be made.
See above for 13, 14 is 9+5, 15 is 3 5s, 16 is 4 4 s 17 is 12 +5, 18 is 14 + 4, 19 is 15 +4 and so on and so on…

July 2018 Brain Puzzler Solution

Q: For how many integer values of x is the value of 4000(2/5)^x  an integer?
A: 9
This can be done with guess and check either by hand or with a calculator.
If x = 0, then 4000(2/5)^0 = 4000 so 0 works.
If x = 1, then 4000(2/5)^1 = 1600   (4000 x 0.4) so 1 works.
If x = 2, then 4000(2/5)^2 = 640     (1600 x 0.4) so 2 works.
If x = 3, then 4000(2/5)^3 = 256     (640 x 0.4) so 3 works.
If x = 4, then 4000(2/5)^4 = 102.4 so 4 does not work.
We need to also try negative integers so here goes:
If x = -1, then 4000(2/5)^-1 = 10000 (4000 x 2.5) so -1 works.
If x = -2, then 4000(2/5)^-2 = 25000 (10000 x 2.5) so -2 works.
If x = -3, then 4000(2/5)^-3=  62500 (25000 x 2.5) so -3 works.
If x = -4, then 4000(2/5)^-4 = 156250 (62500 x 2.5) so -4 works.
If x = -5, then 4000(2/5)^-5 = 390625 (156250 x 2.5) so -5 works.
We have been ok all along because we keep getting even numbers (until x = -4) but now…
If x = -6, then 4000(2/5)^-6 = 976562.5 (390625 x 2.5) so -6 does not work.
There are 9 integers that work:
-5, -4, -3, -2, -1, 0, 1, 2, 3