HOW MANY SIGNIFICANT FIGURES ARE THERE IN THE NUMBER 405,000?

Three, but see below.

Explanation:

You can be certain of three, but that may not be true at all. The only way to know for certain is to put it into scientific notation.

4.05 * 10^5 would be three, but other answers are possible. For example

4.050 * 10^5 would have 4.

if you already have the answer to (a), then just work out how many periods there are in 1 minute, and that's the answer to (c).

b. lifting dumbbell weight at 20-degree angle from the body after a 45-degree angle