The average pot cost is halfway between the costs of the middle two size pots. It is $8.25/6 = $1.375
The largest size pot is 2.5 times $0.25 more, so costs
$1.375 + 2.5*0.25 = $2.00
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The costs of the pots are
0.75, 1.00, 1.25, | 1.50, 1.75, 2.00
where the | represents the average cost, halfway between the middle two costs.