Answer:
$3
Step-by-step explanation:
Let the cost of an eraser, a pen and a ruler be $e, $p, and $r respectively.
Given:
e +2p +r= 13 -----(1)
2e +p +r= 11 -----(2)
3e= 2r -----(3)
(1) ×3:
3e +6p +3r= 39 ----(4)
Substitute (3) into (4):
2r +6p +3r= 39
6p +5r= 39 -----(5)
(2) ×3:
6e +3p +3r= 33 -----(6)
Substitute (3) into (6):
2(2r) +3p +3r= 33
4r +3p +3r= 33
3p +7r= 33 -----(7)
Let's rewrite equations (5) &(7) so it's easier to compare them:
6p +5r= 39 -----(5)
3p +7r= 33 -----(7)
We can eliminate the term p to find the value of r. This can be done by making sure that the coefficient of p is equal in both equations.
(7) ×2:
6p +14r= 66 -----(8)
(8) -(6):
6p +14r -(6p +5r)= 66 -39
Expand:
6p +14r -6p -5r= 27
9r= 27
r= 27 ÷9
r= 3
Thus, the cost of a ruler is $3.
