SOLUTION: he Jurassic Zoo charges ​$7 for each adult admission and ​$4 for each child. The total bill for the 101 people from a school trip was ​$491. How many adults an

 a +  c = 101,   ("a" for the number of adults, and "c" for the number of child)
7a + 4c = 491,

if you want solve it using the system of two equations with to unknowns.


Or simply

7a + 4(101-a) = 491,

if you want to solve it as a single equation.

From the last equation 

7a + 404 - 4a = 491,

3a = 491 - 404,

3a = 87,  --->  a =  = 29.

Answer.  29 adults and 101 - 29 = 72 children.