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