Answer:
$ 5.65
Step-by-step explanation:
let the cost of a pencil and a ruler be $p and $r respectively.
4p +r= 1 -----(1)
6p +3r= 2.10
divide by 3 throughout:
2p +r= 0.70 -----(2)
Q: 3p +13r= ?
(1) -(2):
(4p +r) -(2p +r)= 1 -0.70
4p +r -2p -r= 0.30 (expand)
2p= 0.30 (simplify)
p= 0.30 ÷2 (÷2 on both sides)
p= 0.15
subst. into (1):
4(0.15) +r= 1
r +0.60= 1
r= 1 -0.60
r= 0.40
cost of three pencils and thirteen rulers
= 3p + 13r
= 3(0.15) + 13(0.40)
= 0.45 + 5.20
= $5.65
