Mathematicians

Three and ninety-one ten-thousandths.

Assuming you could average a number per second (not easy if you enumerate some of the larger numbers fully) one billion seconds is 16,666,666 minutes and 40 seconds. It's a little over 31 and 1/2 years, if you never sleep.

8600

Non-linear.

The mean for a single number, such as 6281454562, is the number itself.

It is: 1/4 times 1 = 1/4

The GCF is 13xy.

No.900 * 3 = 2700 so 900 * 30 = 27000 and then 917*33 will be larger still. So it is an extremely poor answer.

Yes and leaving no remainder

It is: 23 as a percentage of 25 = 92%

A city has a higher population density.

No because the associative property can be found in other operations as well.

On the number line it is -17

3 and 5

Pi is a number, a mathematical concept. It doesn't "exist" in a physical sense in the first place (yes, it's the ratio of the circumference of a circle to its diameter, but it also shows up in many surprising equations that are purely numeric and not obviously connected to circles), so it's nonsensical to ask what would happen if it "didn't exist".

If we didn't know about it (which is a very different thing) then ... nothing would happen, other than that we'd have to actually measure the circumference of circles if we wanted to know how big around they were, rather than calculating it from the diameter (which is usually significantly easier to measure).

Forty-two and twenty-three hundredths

Five and thirty-two hundreds.

It is 16666667/1000000.

4 and 40

8 and 20

LCD(22, 33) = 66.

Its evaluation is: (4*5)+(7*3) = 41

you can divide -5/8 by 5 by multiplying -5/8 x 1/5. The 5's cancel and you are left with -1/8.

If a number is 1.2, it would be rounded to 1 because a half is 0.5. If the number is 1.25 it would be rounded to 1.5. If the number is 1.24 it would be rounded to 1.

148/99