Final answer:
By substituting the number of students and adults into the equation 12x + 8y = 240, it is confirmed that 21 students and 6 adults can go to the zoo with a budget of $240, as the calculation equals the budget exactly.
Step-by-step explanation:
To determine if 21 students and 6 adults can go to the zoo with a budget of $240, we use the provided equation 12x + 8y = 240, where 'x' represents the number of adults and 'y' represents the number of children. Plugging in the given numbers:
12(6) + 8(21) = 72 + 168 = 240.
Since the total cost equals the budget, it shows that 21 students and 6 adults can indeed go to the zoo within the budget of $240.