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From the condition, every 6 apples are equivalent to 8 pears.
You just have 12 apples in the basket; the shortage to 18 apples is 6 apples, that are equivalent to 8 pears.
So, you can place 12 apples + 8 pears to the basket.
ANSWER. You can place 8 pears to the basket, in addition to 12 apples.
Solved.