I would use simultaneous equations for this.
let the number of student tickets be s and parents tickets be p.
s + p = 70 . (1)
2.25s + 3.75p = 205.5 . (2)
from (1),
s = 70 - p . (3)
sub (3) into (2)
2.25(70-p) + 3.75p = 205.5
157.5 - 2.25p + 3.75p = 205.5
157.5 + 1.5p = 205.5
1.5p = 48
p = 32
now sub p = 32 into (1)
s + 32 = 70
s = 38
therefore students tickets sold 38 and parents tix sold 32.