To answer your question ("do you multiply the whole number too?"): Yes, you have to multiply the whole number as well. You can't just separate 1/3 from -8 and multiply it with 2/5, because the whole number is part of the mixed #. Whenever you multiply, add, divide, subtract, or do anything to two mixed numbers, you must include the whole term.
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Now your question is addition. Like I said, when you add them together, the whole number and the fractions count as one term.
So basically, -8 1/3 + 3 2/5...
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1) Find the LCM (least common multiple) of the denominators
The LCM of 3 and 5 is 15. To get from 3 to 15, you have to multiply 3 by 5. Now multiply the numbers of the fraction part only by 5.
1/3 --> 5/15 (as you can see, 1x5 = 5, 3x5 = 15)
Now for 2/5: To get from 5 to 15, multiply 5 by 3. Do the same thing to the numbers in the fraction
2/5 --> 6/15 (2x3 = 6, 5x3 = 15)
***TO CLARIFY, DO NOT MULTIPLY THE WHOLE TERM BY THE NUMBER***
Right: 2x3 = 6, 5x3 = 15, 6/15
Wrong: 2/5 x 5 = 2
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2) So now that we found a common denominator, change the two terms to -8 5/15 + 3 6/15 (because we had to find the LCM to make life a lot easier)
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3) We just add them together. Even though they are a term together, you can split them apart and add them together separately thanks to the commutative property of addition*.
Time to split it apart to make life a whole lot easier. -8 5/15 can be split apart into the two terms -8 and 5/15. For 3 6/15, it can be split into 3 and 5/15.
Two options from here on:
Set up a single equation: Put all the terms into one equation. -8 + 3 + 5/15 + 6/15. Solve by adding all the terms together in whichever order you prefer (again, thanks to the commutative property of addition)
It would result in the answer -4 4/15
Or...
Set up two separate equations: Set up an equation for whole numbers, and another one for fractions.
Equation 1: -8 + 3 = -5
Equation 2: 5/15 + 6/15 = 11/15
(6 + 5 = 11)
And we add the sums together. -5 + 11/15 = -4 4/15
And that's it!
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*Commutative Property of Addition: The sum of the numbers will NOT change even if you change the order of the addends
Was an hour late, but I hope this still helped you! If there are any questions, just comment below this