work out the values

2x + 3y = 15.5

x+y = 6        or    x = 6-y    Substitute THIS value of x into the first equation

2 (6-y) + 3y = 15.5

12-2y+3y = 15.5

y = 3.5                          Now sustitue THIS value of y in to the first OR the second equation to find x

x + 3.5 = 6

x = 2.5

2x + 3y = 15.5       (1) 

 x  +  y =   6     ⇒   y = 6 - x     (2)

Sub (2) into (1)

2x + 3(6  - x)  =  15.5

2x + 18 - 3x = 15.5

-1x + 18 = 15.5               subtract  18 from both sides

-1x = -2.5   

x = 2.5

And y =   6 - 2.5 =  3.5

So    ( x , y)  = ( 2.5, 3.5)   

Thank you both!!