* X alone can do 1/20 of the work in 1 day * Y alone can do 1/12 of the work in 1 day * In 4 days, X did 4/20 = 1/5 of the work * The total work is 1 * So the remaining work is 1 - 1/5 = 4/5 * X and Y's daily work is 1/20 + 1/12 = 5/60 * Equating: 4/5 = (Number of days) * (5/60) * Number of days = 20 Therefore, the total number of days to complete the work is 20 days.