This is actually quite simple. Let's think about it!
You start off with $15 and then, you earn another $50 because you did all of your chores. Since you gained $50, that would mean that you have to add $50 onto your $15.
$15 + $50 = $65
After you add, you will end up with $65. After you do you chores, you have to got to the store to buy a shirt that cost $10. Since you are spending money, that means you are decreasing in the amount of $$$ that you have, which means that you have to subtract. You have to take the $10 that you are spending and subtract it from the amount that have already, which is $65.
$65 - $10 = $55
Since you have successfully subtracted, you can see that you will end up with $55 in the end of your transaction.
I hope that I was able to help you enough. :) If you ever need any more help, be sure to follow me. :)
and denote the sector area as 
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(the ratio between area is the same as the ratio between coresponding angles).
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