Complex sentence: According to this theory about 70% of a country's economic development can be predicted by the distance between that country and the equator, and that the further from the equator a country is located, the more developed it tends to be.
Split into simpler sentences: According to this theory, about 70% of a country's economic development can be predicted from the distance between that country and the equator. In other words, the further from the equator a country is located, the more developed it tends to be.