Milton has some books about zoology and 4 times as many books about botany. If he has 80 books total, how many zoology books does he have?
Let z be the number of zoology books and b be the number of botany books. We know that z + b = 80 and b = 4z.
Substituting the second equation into the first equation, we get z + 4z = 80
Combining like terms, we get 5z = 80
Dividing both sides by 5, we get z = 16
#### 16