Each dog eats 6*2=<<6*2=12>>12 cups of food a day
His two dogs eat 12*2=<<12*2=24>>24 cups a day
Thus they eat 24/4=<<24/4=6>>6 pounds a day
So they eat 6*30=<<6*30=180>>180 pounds a month
So he needs to buy 180/20=<<180/20=9>>9 bags of food
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