I must confess we have an advantage with this teaser. Firstly, Mrs R teaches probability at degree level. More importantly, she is a chocolate lover.
Her answer was:
“If you ever buy me a box of 18 chocolates where you get to eat 11 of them, you’ll be in serious trouble!”
Well done Ravvie. And of course, you had best heed the advice of Mrs R !!
I’ll post my answers today or tomorrow, unless Ravvie or someone else would like to do so first.
How about a 2 in 18 (or 1 in 9, or 11.11%) chance of picking a white chocolate praline, and 7 in 18 (38.89%) chance of selecting a chocolate she likes. No doubt Ravvie feels a bit sick after eating his 11.
Mrs R laughing: “that will teach you!”
Ravvie and Steve have nicely cracked this one, and Steve’s explanation is neat (and correct !) 
I’ll add the arithmetic to the table that I posted earlier, just in case it helps.
A Packet of Sweets !
On a day out last Saturday with our two grandsons, they wanted to buy a packet of sweets. They each had some money. However, the younger one was 24p short of the price and the older one 2p short.
Grandma suggested they put their money together, but (surprise, surprise) they still didn’t have sufficient to buy the desired packet of sweets !
How much money did each grandson have ?
Clever one this Don, because at first reading there didn’t appear to be enough info.
I think the younger one had 1p and the older one 23p; the sweets cost 25p.
I decided to do this one in my head, mainly as I have no pen and paper.
If one sweet packet = X, the combined funds are X-24 + X-2 = 2X-26.
Funds < X since they don’t have enough to buy one packet.
Hence 2X - 26 < X, so X < 26
But X > 24 as both have some funds.
This confirms X = 25p, as per @SteveD
That’s exactly how I did it too, though not in my head I must confess.
Mrs R just did it in her head, rather more directly than our method.
She said: “if one grandson was 2p short on his own but still short when including his brother’s funds, then his brother could only have 1p. So they had 1p and 23p and the sweets cost 25p.”
Just what I thought when I first encountered it !
Well done.
Nice explanation Ravvie, and likewise to the method used by Mrs R
Hope you all, including SteveD, Mike_S and others enjoyed it.
It’s been ages … !
My brother-in-law and his sister have a combined age of 77. My brother-in-law is now twice as old as his sister was when he was as old as she is now.
How old is my brother-in-law and his sister now ?
51.3333333 years old for brother in law
25.6666667 years old for his sister
Hi Seakayaker, Good try and I can see where you are coming from, (and your answer has forced me to think this through very carefully in case you are correct), however ….
…. if his sister is currently 25.66 and he is 51.33 then 25.66 years ago (ie, when he was as old as she is now), he would have been 25.66 and his sister would have been 0.
The statement says my brother-in-law is now twice as old as she was then. 2 x 0 doesn’t amount to 51.33
I hope I haven’t goofed with the wording of the teaser …… I have checked it a few times and am pretty confident.
It looks fine to me. I had a race with Mrs R to solve it and for once I won!
We had quite different approaches to get the same answer.
I will post tomorrow night unless someone else gets in first.
Well, I found that quite a teaser and had to resort to trial and error.
So, I have BIL @ 44 and SIL @ 33 = 77. Age difference is 11 years. When BIL was SIL’s current age of 33, sister was 11 years younger at 22 and BIL is now 2 x 22 = 44 years old.
The wording threw me util I realised that it was the age difference that was the key to solving it. Nice teaser.
My solution:
Wording: looks complicated. Key bits likely to be “twice” and as Mike pointed out, age difference. Ignore the rest for now.
Numbers: 77 = 7x11 (factorising is usually helpful). Assume 11 is just an arbitrary multiplier as 7 looks more interesting: 7 = 4+3 and 3 = 2+1. Lucky break as this seems to fit. B=44, S=33 and when B was 33, S was 22.
Mrs R’s solution (I may have got the order wrong as she scribbled it on a receipt when we were at a cafe):
Re-write words and try to understand them.
B+S=77
B=2T
B-S=x
S-T=x
B-S=S-T
B=2S-T=2T
S=3T/2
77=7T/2
T=22
B=44, S=33
The teaser could have used any multiple of 7 for the combined age. If the total age was 84, say, then my factorising approach would have more possibilities so I would have needed to use trial and error to finish off. Mrs R’s approach works for any multiple of 7.
Me too ! Well done Mike. Hope you enjoyed it.
