Puzzle 45
Last week I drove from Aardvark to Beeville.
On the first day, I travelled 1/3 of the starting distance.
On day two, I travelled 1/2 of the remaining distance.
On day three, I travelled 2/3 of the remaining distance.
At the end of day four, after travelling 3/4 of the remaining distance, I was still 5 miles away from Beeville.
How many miles had I travelled so far?
Puzzle Copyright © Kevin Stone
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Hint
Try working backwards.
Answer
175 miles.
The total distance was 180 miles, but as I still had 5 miles to go, the required answer is 175 miles.
Reasoning
Let's try working backwards.
Day 4: I travelled 3/4, which means that the other 1/4 was 5 miles – so 20 miles were left at the start of day four.
Day 3: I travelled 2/3, which means that the other 1/3 was 20 miles – so 60 miles were left at the start of day three.
Day 2: I travelled 1/2, which means that the other 1/2 was 60 miles – so 120 miles were left at the start of day two.
Day 1: I travelled 1/3, which means that the other 2/3 was 120 miles – so 180 miles were ahead at the start of day one.
The full distance was therefore 180 miles, but we were asked how many miles I had travelled so far, and as I still had 5 to go the answer is 175 miles.
Double-Checking
Start distance: 180 miles.
Day 1: I travelled 1/3 of the distance (60 miles), leaving 120 miles.
Day 2: I travelled 1/2 of the distance (60 miles), leaving 60 miles.
Day 3: I travelled 2/3 of the distance (40 miles), leaving 20 miles.
Day 4: I travelled 3/4 of the distance (15 miles), leaving 5 miles.
Puzzle 46
Imagine a chessboard of size 20 x 20 instead of the normal 8 x 8.
How many squares, of any size, are there in total?
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Hint
How many 1 x 1 squares are there, how many 2 x 2, …?
Answer
2,870 squares.
Reasoning
We have squares sizes from 1 x 1 up to 20 x 20, and these are the counts:
1 x 1 = 400
2 x 2 = 361
3 x 3 = 324
4 x 4 = 289
5 x 5 = 256
6 x 6 = 225
7 x 7 = 196
8 x 8 = 169
9 x 9 = 144
10 x 10 = 121
11 x 11 = 100
12 x 12 = 81
13 x 13 = 64
14 x 14 = 49
15 x 15 = 36
16 x 16 = 25
17 x 17 = 16
18 x 18 = 9
19 x 19 = 4
20 x 20 = 1
Total = 400 + 361 + 324 + 289 + 256 + 225 + 196 + 169 + 144 + 121 + 100 + 81 + 64 + 49 + 36 + 25 + 16 + 9 + 4 + 1 = 2870.
Puzzle 47
My local library decided to hand out some of its old books to the 1,400 people who lived in the town.
To each library member, they offered 6 books, and to each non-member, they offered 4 books.
If only half of the members, and three-quarters of the non-members, accepted the books, how many books were given away?
Puzzle Copyright © Kevin Stone
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Hint
As we don't know how of the people were library members, you can try a random number and see what happens.
Answer
4,200 books.
Reasoning
It doesn't matter how many library members there were, as 1/2 of them accepted the books, and each accepted 6 books, which is 3 books per member on average.
Similarly, it doesn't matter how many non-members there were as 3/4 of them accepted the books, and each accepted 4 books, which is again 3 books per non-member on average.
Double-Checking
Both the members and non-members accepted, on average, 3 books each.
And 1400 x 3 = 4200.
Puzzle 48
Here is a curious grid! The sums of the shapes are shown at the end of each row, and at the bottom of each column.
What is the missing sum, and what is each shape worth?
Puzzle Copyright © Kevin Stone
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Hint
Look at the first and third columns first.
Answer
The missing sum is 40.
Circle = 12
Star = 3
Square = 7
Triangle = 9
Reasoning
Looking at Column 1 and Column 3 we have:
[1] Square + Star + Circle + Square = 29
[2] Square + Square + Triangle + Star = 26
If we subtract [1] – [2] we get:
[3] Circle – Triangle = 3
And Row 3 is:
[4] Circle + Circle + Triangle + Circle = 45
If we add [3] + [4] we get:
Circle + Circle + Circle + Circle = 48
So, Circle = 12.
Using [3] we find that Triangle = 9.
Using Column 2 we find that Star = 3.
Using Row 1 we find that Square = 7.
Double-Checking
The rows add to: 35 + 26 + 45 + 25 = 131.
The columns add to: 29 + 36 + 26 + 40 = 131.
The rows and columns must add to the same number, so this could have been used a shortcut to the missing sum.
In total, there are:
5 Squares = 5 x 7 = 35
5 Circles = 5 x 12 = 60
3 Stars = 3 x 3 = 9
3 Triangles = 3 x 9 = 27
Which is also a total of 131.
So all of the totals are the same, as expected.
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