A vat with 700 gallons of beer contains 4% alcohol (by volume). Beer with 6% alcohol is pumped into the vat at a rate of 7 gal/min and the mixture is pumped out at the same rate. What is the percentage of alcohol after an hour

5.2%.

Step-by-step explanation:

After 1 hour (60 / 100) * 700 = 420 gallons of beer have been exchanged.

So we have 420 gallons of 6%  and 280 gallons of 4% after 1 hour.

So the strength at that time =  (420* 6) + 280*4) / 700

= 5.2%.