The gardener has 3 vegetable beds.
Let there be x number of vegetable beds. Then if 3/10 of a bag of mulch is spread evenly over each of her equal-sized vegetable beds, the total amount of mulch used is 3x/10. According to the question, the total amount of mulch used is 9/10 of a bag. Hence,
3x/10=9/10
⇒x=3
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