A World Class Swimmer's Puzzle

A world-class swimmer can swim at twice the speed of the prevailing tide.

She swims out to a buoy and back again, taking four minutes to make the round-trip.

How long would it take her to make the identical swim in still water?

Click here for the answer!

Solution of A World Class Swimmer's Puzzle

What was the puzzle?

 Let C be the speed of water current then the speed of a world class swimmer will be 2C.

When she swims out to a buoy located at a distance say D and back again, the time needed is - 

D/(C+2C) + D/(2C-C) = 4 minutes

D/3C + D/C = 4

Multiplying both the sides by 3, 

D/C + 3D/C = 12

4D/C = 12

D/C = 3  

Now when she swims out to a buoy and back to shore again in still water, she needs only time -

D/2C + D/2C 

Since D/C = 3, then D/2C = 3/2

Hence, 
 
D/2C + D/2C  = 3/2 + 3/2 = 3.

That is, she needs only 3 minutes to swim out to a buoy and back to shore again in still water.

 
 

"How Did They Cross The River?"

Five men and five dogs (each man owned a dog) went hiking. They encountered a river that was swift and deep. The only way to cross it was an abandoned boat, left ashore on their side. But it would only hold three living things. 

Unfortunately, the dogs were edgy and could not be near another person (not even momentarily) unless its owner was present. One of the dogs attended a highly advanced, highly specialized obedience school and therefore knew how to operate the boat (as the men did) -- the other dogs lack this skill. 

How did the five men and the five dogs cross the river?

THIS is how they accepted the challenge! 

The Challenge of River Crossing

What was the challenge?

Let's name five men as M1, M2, M3, M4, M5 & five dogs as D1, D2, D3, D4, D5.
Assume D1 be the dog having advanced skills of operating boat.

TRAVEL CHART :

1] The dog D1 rows D2, D3 across the river and returns back. After returning back D1 takes D4 across the river & returns back once again.

START : D1-M1, D5-M5, M2, M3, M4  DESTINATION : D2, D3, D4

2] Now, M2, M3 and M4 cross river.

START : D1-M1, D1-M5  DESTINATION : D2-M2, D3-M3, D4-M4

3] Someone from DESTINATION needs to return back to allow others at START to cross the river. So, D4-M4 returns and D1-M1 cross the river. 

START : D4-M4, D5-M5  DESTINATION : D1-M1, D2-M2, D3-M3

4] After D1-M1 reaches to the DESTINATION, D3-M3 returns back.

START : D3-M3, D4-M4, D5-M5  DESTINATION : D1-M1, D2-M2

5] Now, M3, M4 and M5 cross the river.

START : D3, D4, D5  DESTINATION : D1-M1, D2-M2, M3, M4, M5.

6] Finally, the dog D1 at the DESTINATION returns back and makes 2 trips to take D3, D4 and D5 across the river.

START : None  DESTINATION : D1-M1, D2-M2, D3-M3, D4-M4, D5-M5.

This way, five men and five dog cross the river successfully.