1. Cost Price (cp) for shopkeeper per apple = total cost of apples / cp per apple = 100/10 = $10 per apple
2. Profit per apple: Selling price (sp) - cost price (cp) = 15-10 = $5/apple
3. Total profit for 100 apples = 100* 5 = $500
4. The total risk (TR) for the shopkeeper if no apples are sold in 7 days is to sell all apples to the farmer at a 10% discount, which is $1 per apple
a total of $100 risk on all apples
How many apples he needs to sell at $5 profit to cover for $100 initial risk = total risk / profit per apple = 100/5 = 20 apples.
Firstly, I do not know if my answer is correct - algebraic expression is TR/(sp-cp).
But here is how I know the answer is wrong: If he sells ONLY 20 apples by the end of 7 days, then the remaining apples (80) will be sold back to the shopkeeper at $9 per apple = 9*80 = $720. Shopkeeper's cost of 80 apples is $800 and $800-$720 = $80.
Gain on 20 apples is $100 and loss on 80 apples is $80. So selling 20 apples is an excessive and I am stuck here trying to find the correct approach and answer.
I'm not familiar with the concept of total risk as you are using it; this is not how I would approach the problem, so I can't be sure where you are making an error in your method (e.g. if your formula ignores the repurchase). Perhaps someone here is familiar with how it is done in business courses.
The equation I wrote just sets the net gain (revenue minus cost plus gain from the discounted repurchase) to zero. I did not calculate the profit per apple, in part because there are two groups of apples, sold and unsold, which would confuse me a bit. I did get an answer smaller than 20.
In fact, one way to get an equation is to do what you did as a check (a very wise thing to have done) as a model for the equation, just replacing 20 with a variable.
I get the answer to be fractional (XX.666...)! Another 'defective question?
I do wish people would take more care when constructing questions like this (to ensure that the numbers 'work out' evenly).
Clearly, under normal circumstances, only whole apples can be sold, so, if the shopkeeper sells one fewer apple than my answer, s/he makes a $4 loss on the operation while if s/he sells one apple more s/he makes a $2 profit. The only way to break even (ie: make "no loss or profit") is to sell someone ⅔ of an apple for $10!
As I see it, it is common in the real world to have the break-even amount
not be a whole number, so I think, if anything, this is a better question than what you envision, because the real world is not as kind as you. (Of course, in the real world, apples don't cost $10 each (not the ones we buy).
What I would actually do is to write an
inequality, which says that we have at least broken even (that is, did not lose money), and
round the solution in the appropriate direction to get a whole number that should be sold. But I don't think it's wrong to say "We will break even if we sell xx.66 apples, so we want to sell at least xy whole apples to make a profit."