If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

### Course: 7th gradeย >ย Unit 4

Lesson 3: Adding & subtracting rational numbers

# Adding & subtracting rational numbers: 79% - 79.1 - 58 1/10

Sal evaluates 79% - 79.1 - 58 1/10. Created by Sal Khan.

## Want to join the conversation?

• I still don't get how are you supposed to covert it into a fraction like you did in the video.
• i don know either i am confused
(1 vote)
• How do you make a fraction a decimal?
• Divide the number on top of the fraction (the numerator) by the number on the bottom of the fraction (the denominator) and multiply by 100% (i.e. move the decimal point two places to the right and add a percentage sign).
• At , Sal factors out a negative sign. Can someone explain to me what that it is and why he did it? Also, can you explain how they became positive? Thanks!
• ๐ ๐๐๐ ๐ข๐๐ ๐๐๐๐๐๐ ๐๐๐ ๐ ๐๐๐๐, ๐๐๐๐ ๐๐๐ ๐๐๐๐๐. ๐๐๐๐๐๐๐๐ ๐๐๐๐ ๐๐๐๐๐ ๐๐๐๐๐๐ ๐๐๐๐ ๐๐๐ ๐๐๐๐๐๐๐๐ ๐๐ ๐๐๐๐๐๐๐. ๐๐๐๐๐ข ๐๐ ๐๐๐๐ ๐๐๐๐'๐ ๐๐๐๐.
• ๐ป๐๐ ๐น๐ ๐๐๐ ๐ป๐ถ๐ธ๐๐๐ ๐๐๐ ๐ถ ๐๐๐๐ถ๐๐พ๐๐ ๐๐พ๐๐?
• Essentially, to factor a negative number, find all of its positive factors, then duplicate them and write a negative sign in front of the duplicates. For instance, the positive factors of โ3 are 1 and 3.
• but why did he put it to adding and not subtracting instead?
• While I would not have done it this way because I have a very good sense of adding and subtracting numbers, the reason Sal factored out a -, thus changing all the signs, is because he saw that the two negatives would give a bigger answer than the positive number, and at this point in 7th grade, more people would do better to subtract a smaller number from a larger number, then just add the negative sign. I would tend to add the two negatives (if you have same signs, you add and keep the sign), so this would give .79 - 137.2, then subtract (opposite signs subtract and take sign of largest absolute value number) to get -136.41.
I really push learning these two addition/subtraction rules with my students over switching things around like Sal did.
• i thought when you change a subtraction sign to a plus sign then the number behind it would become negitive? Please explain this to me because this video just confused me?
• minusing a negative is the same thing as adding a positive
• This is so confusing! Can someone explain it with this question? I'm having a bit of trouble with this one;

-45/40 + 1.875=
• First convert either of the numbers to the same format as the other one. E.g., decimal, fraction.
-45/40 = -1 5/40 = -1 1/8 = -1.125
Now solve the equation:
-1.125 + 1.875 = 1.875 - 1.125 = 0.75
• im confused why did he turn 79% into .79?
• Percent means you divide by 100, so then 79% would be 79/100 which is 0.79. Hope this helps.
• At he talks about "factoring out" a negative sign what exactly is this and what is it used for?
• How do you calculate an addition/subtraction equation that starts with a negitive decimal and the rest are postive decimals?
• To give an example, are you talking about a question like this?
-.325 + .585?
If that is the case, then think of it like this. If the question is adding, and the negative decimal comes first, then do this. .585 -325. That should give you the answer.
If you are multiplying, such as this,
-.325 x .585
it's the same thing as multiplying negative numbers. Multiplying a negative number by a positive number will get you a negative number. So, do this. Take the decimals out, and multiply. You should get this equation.
-325 x 585= -190125.
Now, since you took 6 decimal places out, put 6 in. Your final answer should be -.190125.
If you are subtracting like this,
-.325 - .585,
that 585 will actually ADD to the negative -.325, making it -.910.
If you are dividing, think of it as the same thing as multiplying. Good luck!