1) The half-life is the time required for a substance to reduce to half its initial value. In formulas:

(1)
where
m(t) is the amount of substance left at time t
m0 is the initial mass

is the half-life
In this problem, the half-life of the substance is 20 years:

therefore, the fraction of sample left after t=40 years will be

So, only 1/4 of the original sample will be left, which corresponds to 25%.
2) We can use again formula (1), by re-arranging it:

If we use m(t)=10 g (mass of uranium left at time t), and

(the time is equal to 4 half lifes), we get

So, the initial sample of uranium was 160 g.
A constant velocity implies the two forces must be equal and opposite.
Friction acts horizontal to the ground, therefore we must find the force applied to the sled rope that acts horizontal to the ground.
Do this by resolving:
Force = 80cos53
The force opposing this is equal, and so also = 80cos53 = 48 N (2 sig. fig.)
Unless you are a mutant....I don't think you would make it. I'm 5'8" and most people are 6'7" or less. It's bsically impossible to do. :)
Answer: The statement "The charge cannot be created or destroyed describes the principle of the conservation of charge".
Explanation:
According to the conservation of charge, the charge can neither be created nor destroyed. It can be transferred from one system to another.
In an isolated system, the total electric charge remains constant. The net quantity of electric charge is always conserved in the universe.
Therefore, "the charge cannot be created or destroyed" describes the principle of the conservation of charge.