15a +10c=$225
a + c = 20
Rewrite the second equation so it has a value for c
c= 20 -a
Now using substitution method replace the c for the given value
15a + 10(20-a)=225
15a +200-10a=225
Combine like terms
5a +200=225
5a=25
a=5
So 5 adult tickets were bought and 15 children tickets