1) "30 minutes is equivalent to 0.30 hours". this sentence is wrong. why?
2) the mass of the sun is 1,98x10^30 kg. what is the order of magnitude of this mass?
a) 1,98 kg
b) 30
c) 10^31 kg
d) 10^30 kg
3) the masses of the earth and moon are respectively 5,98x10^24 kg and 7,34x10^22 kg. what is the order of magnitude of their relationship?
4) on the side of a bus stands a large billboard 2,50 m and high 130 cm. calculate the area of the panel. what extent the side of a second panel, square shape, with the same area of the first?

1) This sentence is wrong because 30 minutes is half an hour, and is therefore 0.5 of an hour.
2) I don't quite know how to explain it, but D is the answer.
3)  (Sorry, I can't really help you here. My knowledge on "order of magnitude" is shamefully limited.)
4) Well, we need to convert one of them to the other's unit first. Let's go with cm (centimeters). This would mean 2,5 * 100 = 250. We have 250 and 130 of the same unit, so we multiply to get the area, which is 32500 cm^2. But, this isn't our answer. Now, we have to find the square root of this number to get the side length of a square with the same area. And the square root is: ~180,28. So, the side length of the square is 180,28 cm.