Uneven hourglass

Using only a four-minute and a seven- minute hourglass, measure exactly nine minutes?

It is possible to use multiple hourglasses

#FunFacts #Brainteaser

This discussion board is a place to discuss our Daily Challenges and the math and science related to those challenges. Explanations are more than just a solution — they should explain the steps and thinking strategies that you used to obtain the solution. Comments should further the discussion of math and science.

When posting on Brilliant:

Use the emojis to react to an explanation, whether you're congratulating a job well done , or just really confused .
Ask specific questions about the challenge or the steps in somebody's explanation. Well-posed questions can add a lot to the discussion, but posting "I don't understand!" doesn't help anyone.
Try to contribute something new to the discussion, whether it is an extension, generalization or other idea related to the challenge.
Stay on topic — we're all here to learn more about math and science, not to hear about your favorite get-rich-quick scheme or current world events.

Markdown	Appears as
`italics` or `_italics_`	italics
`bold` or `__bold__`	bold
- bulleted - list	bulleted list
1. numbered 2. list	numbered list
Note: you must add a full line of space before and after lists for them to show up correctly
paragraph 1 paragraph 2	paragraph 1 paragraph 2
`[example link](https://brilliant.org)`	example link
`> This is a quote`	This is a quote
# I indented these lines # 4 spaces, and now they show # up as a code block. print "hello world"	# I indented these lines # 4 spaces, and now they show # up as a code block. print "hello world"

Math	Appears as
Remember to wrap math in `$` ... `$` or `\[` ... `\]` to ensure proper formatting.
`2 \times 3`	$2 \times 3$
`2^{34}`	$2^{34}$
`a_{i-1}`	$a_{i-1}$
`\frac{2}{3}`	$\frac{2}{3}$
`\sqrt{2}`	$\sqrt{2}$
`\sum_{i=1}^3`	$\sum_{i=1}^3$
`\sin \theta`	$\sin \theta$
`\boxed{123}`	$\boxed{123}$

Comments

start 4 and seven together after 4 min 4minhourglass would e empty and 7 minhrglass would have 3 min left , flip 4 in hr glass after 3 min 7min hr glass would empty ,so flip it and after 1 min 4 hrmin glass woul be empty and 7 hrmin glass have 6 min left so flip it and after and wait for 7 min hrglass to empty now nine mins have been calculated

Lalit Goel - 7 years, 2 months ago

Start and stop when any of the glass is empty and calculate time 4 Min>>>0 Min and 7 min>>>0 Min 0 Min>>>4 Min and 3 Min>>>4 Min ------4Min Flip 4 min Hourglass and Continue 7Min Hourglass
3 Min<<<1 Min and 0 Min>>>7 Min ------7Min Continue 4 Min Hourglass and Flip 7Min Hourglass 4 Min<<<0 Min and 1 Min<<<6 Min ------8Min Flip both Now 3 Min>>>1 Min and 0 Min >>>7 Min ------9Min

Brijesh Mishra - 7 years, 2 months ago

Think about this: 44 - 17 == 9. Let us denote the four-minute hourglass by A, the seven-minute one by B. Take 2 A's and 1 B. Use the two A's consecutively and a B in parallel so that upon the completion of the B the two A's contain 1 minute each. Then launch the two A's in succession to measure the remaining 2 minutes.

Serikzhan Kazi - 3 years, 11 months ago

Math	Appears as
Remember to wrap math in `\(` ... `\)` or `\[` ... `\]` to ensure proper formatting.
`2 \times 3`	$2 \times 3$
`2^{34}`	$2^{34}$
`a_{i-1}`	$a_{i-1}$
`\frac{2}{3}`	$\frac{2}{3}$
`\sqrt{2}`	$\sqrt{2}$
`\sum_{i=1}^3`	$\sum_{i=1}^3$
`\sin \theta`	$\sin \theta$
`\boxed{123}`	$\boxed{123}$