siddhantvarma I'll have a crack at it, tell me what you think:
So X is the amount of 15 meter wire the company bought, they paid 3x for it
They sell each 15 meters for $7, so they sell the first 20x (300 meters) for 0.9*7*20 = 126, and they paid 60 for it. You can instinctively tell that it's above 40%.
With these questions I always plug in the middle answer and check if it's higher or lower:
1200 meters = 80x.
300 = 0.9 of the original price
600 = 0.85
900 = 0.8
1200 = 0.75 = 3/4
so calculate 80*(3/4)*7 = $420, and it cost the company $240. 24/42 = ≈0.57, or 1-0.57 = 0.43*100 = ≈43% profit margin.
This means that D, 1500, will probably be a bit less than 40% margin, but we can check anyway:
1500 = 0.7*100*7 = $490, and it cost the company 3*100 = $300. 300/490 = 60/98, which again intuitively means that it's a bit more than 60/100. 1-(61/100) = 39% margin, which is below the 40% margin that the company wants.
Thus, the company can sell more than 1200 but less than 1500. Answer
DWith this sort of questions I estimate a lot so let me know if this method is not correct.