Q: Here is the Math/logic brain teaser that took the Internet by storm in mid-April 2015:
Albert and Bernard just became friends with Cheryl, and they want to know when her birthday is. Cheryl gives them a list of 10 possible dates: May 15 May 16 May 19 June 17 June 18 July 14 July 16 August 14 August 15 August 17
Cheryl then tells Albert and Bernard separately the month and the day of her birthday respectively. Albert: I don’t know when Cheryl’s birthday is, but I know that Bernard does not know too.
Bernard: At first I don’t know when Cheryl’s birthday is, but I know now.
Albert: Then I also know when Cheryl’s birthday is.
So when is Cheryl’s birthday?
A: July 16th.
Can’t be a unique day. Therefore it cannot be May or June.
July 14 July 16
August 14 August 15 August 17
Has to be unique
July 16
August 15 August 17
If Bernard know and Albert now knows that it has to be July 16th. If it was August then Albert would not know.
(Thanks to Adam Schwartz for this solution!)
Q: If a grandfather clock takes five seconds to strike 6 o’clock, how many seconds will it take to strike 12 o’clock?
A: 11 seconds
Eleven. It will always be one second short of whatever the o’clock since the time doesn’t start until the first strike.
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When can you add 2 to eleven and get 1?
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on a clock
you can add 2 to 11 and get 1
except if you have a clock like this for military/overseas time:
Q: Tree A and tree B both have some birds. The tree A birds tell the tree B birds “if one of you comes to our tree, our population will be the double of yours.” The tree B birds tell the tree A birds “if one of you comes here, then our population will be equal to that of yours.” How many birds in each tree?
A: Tree A has 7 birds and tree B has 5 birds
This can be done with guess and check or algebra
This is for statement 1) A + 1 = 2(B – 1)
This is for statement 2) A – 1 = B + 1
This is A in terms of B from 2) A = B + 2
Then sub into statement 1 and solve for B
B + 2 + 1 = 2B – 2
B + 3 = 2B – 2
B = 5
then can solve for A using either equation A – 1 = 5 + 1 therefore A = 7
Math Confidence by Robin the Math Lady 