Well, here is what I called the 'solution'. Not sure it's right. But it proved a useful tool to give to (CPL) students. It wasn't the answer that was important, it was the things they thought about on the way.
Feel free to borrow/edit/pass off as your own.
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Solution
Standard lapse rate is 2 degrees c per 1000 ft.
Hence temperature at B is 21 – (2 x 8) = 5 degrees C
Pressure altitude at B is (29.92 – 29.52) x 1000 = 400 ft.
7600 ft + 400 ft = 8,000 ft
Total distance to fly is 120 nm.
This would take 1.7 hours at 70 kts. 1.7 x 9 gallons per hour = 15.3 gallons.
20 minutes fuel is required by law (see FAR 91.151). This is 3 gallons. Add 2 gallons for startup/shutdown.
Total fuel required is 20.3 gallons. Fuel weighs 6 lbs per gallon.
Hence Total fuel weight is 20.3 x 6 = 121.8 lb
See In ground effect chart (POH page 5-5):
Max gross weight at B is 1300 lbs. (8000 feet pressure alt and 5 degrees c)
1300 - 890 - 130 *- 121.8 = 158.2 lbs of gold can be carried back to B
158.2 / 11 = 14 gold bars
BUT…. Max allowable under each seat is 50lbs.
So you can only carry… 100/11 = 8 gold bars. (4 under each seat)
….unless you put more gold on the left seat – but is this legal??