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Course: Math (NSDC) - English>Unit 3

Lesson 8: Multiplying fractions

Multiplying fractions word problem: bike

This video is all about understanding how to multiply fractions and mixed numbers. Watch as the steps are explained in a simple and fun way. Created by Sal Khan.

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• isn't 3 1/3's improper fraction supposed to be 10/3
• Hey, the answer is 10/3 but he just did 9/3 + 1/3 to simplify how to find ten thirds here
• I didn't get really why we should multiply! I do on paper though. But Sal understand that right way! Can someone explain that why should we multiply?
• As you learn word problems, you will find that there are a variety of formulas that occur and that you need to learn. This problem uses one of those formulas. Specifically, it used the formula: Distance = Rate (Time).

Sal knows this formula, so he knows he needs to multiply the speed/ rate (the 1/5 miles per minutes) times the time (the 3 1/3 minutes).

Hope this helps.
• umm, why did he not simplify the 10 and the 5 before multiplying them? that would have make things much easier
• He could but maybe it would be easier for some people if he does the other way.
• what if we divide what will we get ?
• at to about he multiplies 1/5X 3 + 3X like 1/3. How is this possible?
• Wait so I don't get it is this how u do it? for example 1/5 x 3/4 so first you multiply the numerator with the numerator which is 4 x 5= 20 and then you multiply the dinomerator's which is 1 x 3= 3 so then the answer would be 3/20? Is that right?
• You are right but I think you got confused with the numerators and denominators. 4 x 5 is the denominator and 1 x 3 is the numerator. I hope this helps!!
(1 vote)
• Is it 9/3 or 10/3? I know the answer is 10/3, but did Sal mean to write 9/3? I mean, was 9/3 his answer, or was 10/3 his answer?
(1 vote)
• I assume you are referring to the section at about in the video. Sal is converting 3 1/3 into 10/3. He wrote it out in steps. 3 1/3 = (3*3)/3 + 1/3 = 9/3+1/3 = 10/3. He writes the result of 10/3 at about in the video. The 9/3 is just an intermediate work result.
Hope this helps.
• Is there a faster way?
anybody
• The second way shown in the video above is definitely faster; the first way definitely used a bit of algebra. You turn the mixed number (3 1/3) into an improper fraction (10/3), and then you multiply the numerators and denominators together. Then simplify your answer, also as shown above. It won't take you long after you do it several times.
• I am having problem with understanding the wording and the relationship with the numbers Comments following use the "plug and chug" method of D= R×T but which numbers go into where this is not as intuitive to me thats why I have always had problems with understanding maths and world application especially algebra eg. She has × apples and y oranges find y for how long it would take to bake an apple pie Vs making a glass of Orange juice ! ? the question given by Sal here are typical Of the reason for why I disliked maths my brain would instantly go foggy like the numbers and words were duelling it out with no winners but my patience losing out! please I need help there is something very wrong with my brain ! many have said (and I think that as many have thought) that i was retarded and/or dropped on my head! Perhaps....
(1 vote)
• Don't worry... you're not the only one who feels this way about algebra :)
When your brain fogs up, I would suggest you take a cold drink of water or iced tea and maybe splash some cold water on your face? I would also suggest maybe taking a walk outside (of course, you can't do any of this in school). If you have money to spare you can buy a fidget toy to help concentrate: https://www.amazon.com/s/ref=nb_sb_noss_2?url=search-alias%3Daps&field-keywords=fidget+toys.
Hope I helped!! :)