**Sara** needs to trek from an **oasis** to a** destination** **10** miles away across a barren desert.
**The facts: **
- Crossing
**one mile** of desert requires using **1 gallon** of water.
- Sara can only carry
**6** gallons of water at a time.
- Sara can drop a water
**cache** (of any amount of water from the supply
she is carrying at that moment) at any of the nine stops along the
route, and then pick up any part of the cache on a later trip.

What's the **minimum** number of times Sara must leave the oasis in order to cross the entire 10 mile span of desert?
**This is how she optimizes her journey! **

**What was the challenge in journey?**
1. First Sara collects **12 gallons** of water at **milepost 1** after having **3** trips from source. She uses 2 gallons (out of 6) for forward & backward journey from source to milepost & dropping 4 gallons in cache at milepost 1.

2.She collect 6 gallons more water at the start of 4th trip from source & drops 5 gallons at milepost 1. Now, she doesn't need to return back to source and **17 gallons** of water available at **milepost 1.**
3.In next 2 rounds, she moves** 8 gallon**s of water from milepost 1 to** milepost 2** (1 for forward + 4 for drop + 1 for backward journey in each round).

4.Now only **5 gallons** left at milepost 2. She uses 1 gallon for journey from milepost 1 to milepost 2 and drop remaining 4 gallons at milepost 2. Now, **12 gallons** of water is available at **milepost 2.**

3.Next, using 2 gallons (out of 6 which is maximum she can carry) she moves from **milepost 2** to **milepost 4** and drop 2 gallons at **milepost 4** & comes back at milepost 2 using remaining 2.
4. Again, on arriving back at milepost 2, she has left with **6 gallons** of water at** milepost 2 **out of which she uses 2 to reach milepost 4 where 2 gallons of water still available there already collected in previous round. Now, she doesn't need to return back from
milepost 4.
5. She uses the remaining **6 gallons **of water to reach at the** milepost 10.**
To conclude, Sara has to leave Oasis **only 4 **times as describe in steps 1 and 2 if she want to cross the entire 10 mile span of desert.

A dead body lies at the bottom of a multistory building. It looks as though he committed suicide by jumping from one of the floors.

When the **detective** arrives, he goes to the first floor of the building, opens the closed window, and **flips** a coin towards the floor. He goes to the second floor and does the **exact** same thing. He continues to do this until he gets to the **top** floor of the building.

When he comes back down, he states that it was** a murder** and not a suicide. How does he know that?

(Entry to the terrace was banned due to some ongoing work).

**Read the detective's logic in the case!**
**What is the case?**
Obviously, the person can't jump with window **closed** or come outside of the window & close that window from inside.

The **detective** checks if windows is closed from inside by opening window & **flipping** the coin toward the floor. He flips the coin to** mark the count** of that particular floor where window was closed from inside & he has to open it to flip the coin.

At the end, he collects as many coin as floors of that building. So he concludes that no floor had **open** window from where the person might have jumped.

So, he concludes that it was **the murder** and not the suicide.** **