Let s be the number of gallons of seed Carson uses and f be the number of gallons of fertilizer. We know that s = 3f and s + f = 60.
Substituting the first equation into the second equation produces 3f + f = 60
Combining like terms, we get 4f = 60
Dividing both sides by 4, we get f = 15
Substituting that value into the equation s = 3f, we get s = 3 * 15 = <<3*15=45>>45
#### 45