Each person has to pay 278.4/(2 *15+20+30) = 278.4/80 = 3.48 yuan per day.

After living for half a month, everyone paid 3.48* 15=52.2 yuan.

After living for 20 days, everyone paid 3.48*20=69.6 yuan.

If you stay for one month, everyone will pay 3.48*30= 104.4 yuan.

Checking calculation is 52.2*2+69.6+ 104.4=278.4.