The wording "onions cost twice as much as potatoes" is understood to mean the cost per pound of onions (n) is equal to two times the cost per pound of potatoes (2p). Then the appropriate equation would be ...
2p = n
Then the solution is ...
6p +3(2p) = 18
12p = 18
p = 18/12 = 1.50
n = 2p = 2(1.50) = 3.00
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The equation should be 2p = n; onions cost $3.00 per pound; potatoes cost $1.50 per pound.
