Question 509600
Adult tickets	x	numbers						
Child tickets	y	numbers						
y-x=8								
-1	x	+	1	y	=	8	.............1	
12	x	+	7.5	y	=	138	.............2	
Eliminate y								
multiply (1)by 		-7.5						
Multiply (2) by		1						
7.5	x		-7.5	y	=	-60		
12	x		7.5	y	=	138		
Add the two equations								
19.5	x				=	78		
/	19.5							
x	=	4      			
plug value of 			x	in (1)				
-1	x	+	1	y	=	8		
-4		+	1	y	=	8		
			1	y	=	8	+	4
			1	y	=	12		
				y	=	12
Adult tickets	4    	numbers						
Child tickets	12    	numbers						
m.ananth@hotmail.ca		

If you want to solve with one variable
let adults tickets be x numbers
children 's tickets = x+8

12*x+7.5(x+8) = 138

solve for x