At the local nursery school, I was chatting to my daughter's friends and noticed a number of things.
Frankie has mousy coloured hair and the girl with black hair was wearing a green t-shirt. Lucy is not blonde and Lauren does not have brown hair, Chloe was wearing a blue t-shirt. The blonde girl was not wearing red and Lauren was not wearing green. I can't remember who was wearing a yellow t-shirt.
Can you work out the colours of the t-shirt and hair for each child?
Reasoning
Since no digit is duplicated, neither number can end in 1, otherwise, the last digit of the answer would already have been used.
Neither number can end in 5 because the answer would then end in 5 or 0.
So the first number can only be 62, 63, 64, 67, or 68.
We can now look at what the second number can end with, and we find that …
if the first number was 62, the second number can only end in 4 or 7.
Why …
not 1 as previously explained
not 2 because we've already used that in the 62
not 3 because 62 x *3 would end in 6, which is already in the 62
not 5 as previously explained
not 6 because we've already used that in the 62
not 8 because 62 x *8 would end in 6, which is already in the 62
We can repeat this for the other possible first numbers and find that …
if the first number was 62, the second number can only end in 4 or 7.
if the first number was 63, the second number can only end in 4, 7, or 8.
if the first number was 64, the second number can only end in 2, 3, 7, or 8.
if the first number was 67, the second number can only end in 2, 3, or 4.
if the first number was 68, the second number can only end in 3 or 4.
Let's check these in turn …
If the first number was 62, the only possible values are:
62 x 14 = 868
62 x 34 = 2108
62 x 54 = 3348
62 x 74 = 4588
or
62 x 17 = 1054
62 x 37 = 2294
62 x 57 = 3534
62 x 87 = 5394
Only 4 calculations are required for each option, as we didn't need to check 62 x 84 as we already know that the answer would end in 8, or 62 x 47 as we already know that the answer would end in 4.
All of these answers fail as they don't contain all of the digits.
Similarly, we can look at 63, then 64, etc. For each of the numbers, we have to check the possible endings, but in each case, only 4 calculations are required (as seen in the examples above).
From the first statement (we take one off as Billy is a boy):
B - 1 = G
and similarly:
2 x (G - 1) = B
Substitute G from the first equation into the second equation to give:
2 x ((B - 1) - 1) = B
2 x (B - 1) - 2 = B
2 x B - 2 - 2 = B
2 x B - 4 = B
B = 4
which means that B = 4 and hence G = 3.
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