If 15 pen cost $3.90, which equation would help determine the cost of 6 pens?
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5x -2y = -10 .....1
3x + 6y =66......2
on multiplying eq. 1 by 3
5x -2y = -10 .....1 ×3
15x -6y =-30
now subtract both equations ,
15x -6y =-30
3x + 6y =66
-___-_____-_
12x =-96
x= -8.
putting value of x in eq 1
5×8 -2y =-10
40 -2y =-10
40-10=2y
50=2y
25=y.