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.