This is very simple if you think about it. Ill explain it without confusing your mind with mathematical jargon, because I understand you looked this up for a reason.
You have to use your choices for this.
A: 6
B: 28
C: 32
D:34
Keep those numbers in mind.
She uses a total of 38 Bills.
Lets think of the first letter choice.
Letter Choice A:
If she uses 6 five-dollar bills ($30) she still needs to pay $190 more.
She would need 19 ten-dollar bills to pay this.
But then she would only have used 25 bills in total. It is stated she used 38.
Letter Choice B:
If she uses 28 five-dollar bills ($140) she still needs to pay $80 more.
She would need 8 ten-dollar bills to pay this.
But then she would only have used 36 bills in total. It is stated she used 38.
Letter Choice C:
If she uses 32 five-dollar bills ($160) she still needs to pay $60 more.
She would need 6 ten-dollar bills to pay this.
That would make it so that she used 38 bills, making this our answer.