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The question is "What's Stella's opportunity cost of working on her serve for an hour?" The rest of the info looks like it's meant to confuse you. They charge $10 an hour. She values an hour of her time at $20. I believe that means she would pay $20. Hope this helps!
Answer:
a) Interest earned = $36
New Balance = $336
b) Interest rate = 0.05 or 5%
New Balance = $517.5
c) time t = 5
New Balance = $612.5
d) Principal Amount = $675
New Balance = $783
Step-by-step explanation:
We are given:
a) Principal (P) = $300
Rate (r) = 3% or 0.03
Time (t)= 4 years
Interest earned = ?
The formula used is: 
Putting values and finding interest

So, Interest earned = $36
New Balance = Principal + Interest = 300+36 = $336
b) a) Principal (P) = $300
Rate (r) = ?
Time (t)= 3 years
Interest earned = 67.50
The formula used is: 
Putting values and finding rate

So, Interest rate = 0.05 or 5%
New Balance = Principal + Interest = 450+67.50 = $517.5
c) Principal (P) = $500
Rate (r) = 4.5% or 0.045
Time (t)= ?
Interest earned = $112.50
The formula used is: 
Putting values and finding time

So, time t = 5
New Balance = Principal + Interest = 500+112.50 = $612.5
d) Principal (P) = ?
Rate (r) = 8% or 0.08
Time (t)= 2 years
Interest earned = 108.00
The formula used is: 
Putting values and finding Principal

So, Principal Amount = $675
New Balance = Principal + Interest = 675+108 = $783
36-40= -4
36 subtracted by 40 equals negative 4
-4