128 gallon tank salt 2%, water evaporated salt to 4% add how many gallons need to be added

Question

The hippo tang fish at the zoo requires the 128 gallon tank to be 2% salt. After period of time, some water evaporated, raising the salinity to 4%. How many gallons of water must be resupplied in order to bring the salinity back down to 2%?

Answer 1

128 gallons of a 2% salt solution

2%*128=0.02*128=2.56 gallons of salt in the 128 gallons of solution

2.56/128=2%

2.56/n=4% where n=# gallons of solution that you have now

the water evaporated but the amount of salt stays the same

2.56/n=4/100

cross multiply

100*2.56=4*n

256=4n

n=256/4

n=64 gallons of a 4% salt solution is what you have now

you lost 64 gallons of water

you must add 64 gallons of water to the 64 gallons of the 4% salt solution

now you have 128 gallons of solution and 2.56 gallons of this solution is salt

2.56/128=0.02=2%

again, you must add 64 gallons of water to the 64 gallons of the 4% solution to get back down to a 2% salt solution