Hi officiisnobis,Good news: your setup is already spot on. The trial-and-error can be dropped entirely with
one observation you almost had.Write total profit as a single expression in
one variable. Using your notebook count x:
Profit = 5x + 8(140 - x) = 1120 - 3xThis is a straight line that
decreases as x grows. That's the whole trick: because profit moves in one direction with x, you never have to test random cases - you only ever check the
endpoints of the range each statement allows.
Statement (1)You got x >
86.15, so x ≥
87. Since profit falls as x rises, the
biggest possible profit sits at the
smallest x:
- x =
87 - profit =
1120 - 261 = 859Even the best case is under
900, so every case is under
900. Definite
No -
sufficient. No case-hunting - just plug the boundary.
Statement (2)Here x >
56, so x ranges from
57 up to
140. Check the two ends:
- x =
57 - profit =
1120 - 171 = 949 (Yes, >
900)
- x =
140 - profit =
1120 - 420 = 700 (No, <
900)
Two endpoints, two different answers -
not sufficient.The takeaway: once profit (or any quantity) is a
monotonic expression in a single variable, the extreme answers always live at the ends of the allowed interval. Test the boundaries, not scattered guesses - that's what turns this into a
30-second problem.
So the answer stays
A, reached with zero guessing.
Answer: Aofficiisnobis
This took a lot of time in testing cases. Is there a faster way to do it?
What I did -
We know N+P=140
Notebook - Cost - 30 ; Sell - 25; Profit - 5
Planner - Cost - 20 ; Sell - 12; Profit - 8
1. 5N>8P
5N>8(140-N)
N>86.something
After testing cases. Sufficient
2. 30N>20P
30N> 20(140-N)
N>56
Testing cases it shows different results. Not sufficient
Answer A