Let's call:
a: number of adults
c: number of children
90 people visited a museum, then:
a + c = 90
Tickets to the museum cost $12 for each adult and $7 for each child and the museum collected $950, then:
12a + 7c = 950
Isolating c in the first equation:
c = 90 - a
Replacing it into the second equation:
12a + 7(90 - a) = 950
12a + 7(90) - 7a = 950
5a + 630 = 950
5a = 950 - 630
a = 320/5
a = 64
64 adults visited the museum
