Complex sentence: Consider a system in which voters can vote for any candidate from any one of many parties; suppose further that if a party gets 15% of votes, then that party will win 15% of the seats in the legislature.
Split into simpler sentences: Consider a system in which voters can vote for any candidate from any one of many parties. Suppose further that if a party gets 15% of votes, then that party will win 15% of the seats in the legislature.