If the purchase price of a commodity is reduced by 5% while the selling price remains unchanged, the profit rate can be increased from the current a% to (a+15)%, what is the value of a? (Explanation: Profit rate, that is, the percentage of the amount of money higher than the purchase price to the purchase price.)
如果某商品进价降低5%而售价不变，利润率可由目前的a%增加到(a+15)%，则a的值为多少？(说明：利润率，即高出进价的钱数占进价的百分比．)
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