SOLUTION: What should be added to (-7/20) to get (-2/5)?

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Question 754303: What should be added to (-7/20) to get (-2/5)?
Found 2 solutions by Alan3354, mgmoeab:
Answer by Alan3354(69443) About Me  (Show Source):
You can put this solution on YOUR website!
What should be added to (-7/20) to get (-2/5)?
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(-7/20) + what = (-2/5)
Subtract (-7/20) from both sides
what = (-2/5) - (-7/20)
Get a common denominator
what = (-8/20) - (-7/20)
- times - = +
what = (-8/20) + 7/20
Combine
what = -1/20

Answer by mgmoeab(37) About Me  (Show Source):
You can put this solution on YOUR website!
Proceed as follows:
1. What are they asking you?
You need a NUMBER that once you add -7%2F20 to it, you get -2%2F5
Now, let X be the number you dont know.
The question can be written as:
X+%2B+%28-7%2F20%29+=+-2%2F5 (a number plus %28-7%2F20%29 gives me %28-2%2F5%29
SOLUTION:
X+%2B+%28-7%2F20%29+=+-2%2F5
This is the same as:
X+-%287%2F20%29+=+-2%2F5
If you solve for X, you'll get:
X+=+%28-2%2F5%29+%2B+%287%2F20%29 (Notice how you have a negative (7/20) on the left side, and it turns positive on the right side)
X+=+%28-2%2F5%29+%2B+%287%2F20%29
X+=+%28-1%2F20%29
This tells you that if you add %28-1%2F20%29 to %28-7%2F20%29 you'll get %28-2%2F5%29