Why are most of the capitals of Europe on major rivers?

Most of the world's major cities were built on or around areas of freshwater. Our ancestors chose to settle near these areas as rivers were a means of supplying drinking water for their families and beasts, as a food supply, used for irrigating crops, and as a means of transport in order to aid commerce.

an angle + its complement = 90 degrees

the precambrian aged rocks could be found in australia. australia is a region that has a lot of ancient civilization history from million years ago. in this region, there are a lot of recognizable fossils buried in its land which is considered a precambrian-aged rocks