To do that, I can divide the total number of memos by the number of typists and the number of days. That gives me 600 / (4 * 3) = 50 memos per typist per day. Now, if I have three typists and one day, I can multiply the rate by the number of typists and the number of days. That gives me 50 * 3 * 1 = 150 memos. 
Four typists can type a total of 600 memos in three days. At that rate, how many memos can three typists type in one day?