D=60. multiply both sides of the equation by 4 since it’s a factor, giving you 2d-20-d=40. since there are like terms with d, collect them to get d-20=40 and move 20 to the right and change it’s sign. that will give you d=40+20, then calculate to get d=60. hope this helped! :)
the first fish tank has 100 gallons of water and 1 gallon is removed every hour which makes 100-h
the second tank has 40 gallons in it and 2 gallons are added every hour which makes 40+2h
after 20 hours the second tank will have more gallons than the first tank
this means that 40+2h would be greater than 100-h since it's also asking you "After how many hours will the first tank have less water than the second?"
