You first have to find out what 4/5 of a 30 gallon tank is so you can do that two ways. For me the easiest way is to convert 4/5 to a percent which is 80%. You then multiply the 30 gallon tank by 80% and you get 24 gallons. You can also multiply the 4/5 times 30 or 4/5 x 30 by multiplying the numerator 4 times the other numerator 30 and get 120 and then divide by 5 which also gives you 24 gallons. The problem says it takes 6 seconds to fill a half a gallon so you multiply the 6 seconds by 2 and get 12 seconds per gallon. You then multiply 12 seconds times 24 gallons or 12 x 24 = 288 seconds and then divide that by 60 seconds per minute and you get 4 minutes and 48 seconds.
