math question

Question

I have a 500 gallon tank at 80%. If I remove 100 gallons would the tank be 60%?

Answer 1

Jim,

 

What is 80% of 500?  Remember that percent means for each 100, so 80/100 = x/500. Cross multiply and we get that x = 400 gallons, so our tank has 400 gallons currently. 

 

We can do the next step multiple ways, but let's subtract the 100 gallons to get 300.  Now, 300/500 = x/100.  Cross multiply again and we get that 300 gallons is indeed 60%.

Answer 2

If the tank holds 500 gallons, then at 80% (0.80) capacity, it holds:

 

500*(0.80) = 400 gallons

 

So if I remove 100 gallons, I have 400 gallons - 100 gallons = 300 gallons left:

 

500*x = 300

 

x = 300/500

 

x = 0.60 = 60%