17500000000mg = 38580lb 14.3350oz
Why not?
Both accurate to 11 places.
Very much as an engineer would do it.
No they aren't and no they wouldn't.
These are approximations good to 1 or 2 digits. For their answer to be true in these calculation there would have to be exactly 3.5 billion bulbs and each to average 5.000 000 000 0 mg of mercury. The first isn't true and the second isn't true.
All of these are approximations good to one or two significant digits. The answer 17 500 000 000 mg is good to a few digits (even that is too many), so it is ridiculous to express the weight to the nearest 100,000th of an ounce.
The best answer would be "about 40,000 pounds" assuming their original numbers were reliable.
People think giving answers with many decimal places makes them look very smart and exacting. The contrary is true.
Any high school science teacher can tell you how fun it is to try to convince a kid that most of the digits in his chemistry lab answer of 3.547 024 931 grams (delivered by his $100 calculator) are meaningless.
Another example is when people say things like, "The speed limit is 60 mi/h here or 96.560 640 km/h in Canada." The Canadian sign would say 100 km/h. This all reminds me of the story of the kid who asked a museum guard how old a dinosaur skeleton was. The guard replied that it was 50,000,006.5 years old. The kid asked how he knew it was that old and the guard replied that they told him it was 50 million years old when he was hired and that he had been working there for six and a half years.