Context: Population has outstripped the supply of freshwater, usually from rainfall. The northern atolls get 50 inches (1,300 mm) of rainfall annually; the southern atolls about twice that. The threat of drought is commonplace throughout the island chains.

Question: How many millimeters of rain do the northern atolls of the Marshall Islands receive?
Answer: 1,300

Question: What is the main source of fresh water for the Marshall Islands?
Answer: rainfall

Question: What is the Marshall Islands often threatened with?
Answer: drought

Question: How much more rain do the southern atolls get compared to the northern?
Answer: twice