Hello Ravneet.
Lets say a represents # of adult tickets;
c represents # of children tickets;
a +c =750
2.5a +1c = 1162.5. (both equations have 1c, if we multiply the first equation by -1 and add these equations we can cancel c)
-a -c = -750
2.5a +c =1162.5
1.5a = 1162.5 -750 = 412.5
1.5a = 412.5
a = 275. they sold 275 adult ticket
the total was 750; they sold 750 -275 = 475 children ticket
hope that helps