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### Course: Algebra (all content)>Unit 2

Lesson 12: Old school equations with Sal

# More percent problems

Slightly harder percent problems. Created by Sal Khan.

## Want to join the conversation?

• Alright now I'm totallly confused. How is 125 multiplied by 0.75 = 94.75
I tried it over and over again but I got a different answer. Am I doing my multiplication wrong???

• He made a little mistake. 125 x 0.75 = 93.75
• How do you solve this problem? This type wasn't in the video:
Gabriela has 67 dollars in the bank today. Yesterday, she had 163 dollars in the bank. By what percentage did Gabriela's bank account decrease over the past day? (Round your answer to the nearest hundredth of a percent.)
• If you subtract 67 from 163 you get 96 which is the amount it decreased by. So 163 times what percentage equals 96?

163(x) = 96

is a good equation to use. From there you get

x = 96/163

which is .58895 or 58.90% when written as a percentage rounded to the nearest hundredth of a percent.
• For the first problem, why is the answer not \$62.50? Where I have \$50 but it is worth 25% more for every dollar spent - 50+(50*0.25)=62.50? I know this is wrong - I just don't understand why it is wrong...
• Hi Matt! You started by reformulating the situation as : "My dollars are worth 25% more" thinking that it is equivalent... It is not. Of course you thought that 25% of 50 dollars is the same as 50 times 25% of 1 dollar, but 25% of 50 dollars is not the same as 25% of, say, 62,50 nor 66.67...
Start instead by thinking "There's this value That I'll call X, which minus 25% of itself, equals 50." Then "solve" for X. Hope this helps! :)
• If you only have \$50 to spend and the sale is allows you to spend a max of 66 and 2/3 or \$66.67. You can't round up, you must round down to stay inside the \$50 budget to \$66.65. Unless however they have a penny tray to cover the extra cent.
• I agree with your reasoning. My only point is that if you round 66.666 (repeating decimal) down, you get \$66.66. Which at 25% off would be equal to \$49.995, a number, even if rounded up, is still less then (or equal to) \$50.
• so at , sal said that when something shrinks by 25%, it is now 75% of the original, does that apply to any other similar problem,? like in 2,576 decreased by , say, 10%? that's just 90% of the original 2,576, right? which would be 257.6?
• Almost, you have one little error. If you have 2,576, then 10% is the 257.6. 90% would be 2,318.4

Otherwise, you have the right idea.
• what grade is this ulimited by i mean like what grade level is this best for
(1 vote)
• Well, it's not necessarily meant for any grade level. It's meant for people who have a good understanding of arithmetic and pre-algebra.

But I'll be direct and say that in the U.S. it's typically taught in 6-8th grade, but you can learn it no matter what grade your in.
• i have 4,396,998. and i got 85,000,000 more how much do i have now?
(1 vote)
• Hi everybody,
I need your help to figure out how t o solve operations like this...your help is appreciated guys...
In each of the following examples there are two unknowns. One of the unknowns will be defined as “x”. Use your knowledge of key words/phrases to describe the other unknown in terms of “x”. The underlined portion of each sentence represents the translation you must make. The words in bold letters will be replace by “x”.
A) the first of two angles is equal to Twice the second angle less five.
Second angle: x, First Angle:
B) The first price is the sum of the original price and 7% of the original price
Original price: x, Final price:
C) The length is 7 more than the product of the width and 4.
Width: x, length:
d) a smaller number is the same as 3 less than the quotient of a larger number and 8.
Larger number: x, smaller number:
(1 vote)
• Redberly,
I can show you how to solve a similar problem.

E) The length is 12 less than the product of the width and 9.
Width: x, length:

You want to convert the words to mathematical symbols.
First replace the word "width" with your x as instructed
E) The length is 12 less than the product of the x and 9.
Let's use L for the other unknown so put L in for "length"
E) The L is 12 less than the product of the x and 9.
The word "is" means equals so put "=" in for "is"
E) The L = 12 less than the product of the x and 9.
"The product of x and 9" means "x times 9"
E) The L = 12 less than x * 9
"12 less than" means subtract 12
E) The L = x * 9 - 12
E) L = x * 9 - 12
Now read the instructions again. You need to "describe the other unknown in terms of x" so you need to solve for L
And L is already by itself on the left side of the equation, so it is solved for L.
L= 9x-12
or
length = 9x-12

That should help you do the very similar part C of your problem. And it should help you understand what you are trying to do with part A B and D.

I hope that is of help to you.
• For the 25% off problem; shouldn't you round down rather than up? Since rounding up would put you slightly over the 50 dollars.
(1 vote)
• I agree, but rounding up means 75% more of one half cent at most which is 0.375 cent. The amount is neglectable and the store would have to round down.
66.67 * 75% = 50.0025 which will round down to 50.00
But he forgot to include sale taxes :p