For a one-digit decimal, multiply its decimal part by 2 times, the number is 7.8; expand its decimal part by 5 times, the number is 10.5. How much is this number?
一个一位小数，把它的小数部分扩大2倍，这个数是7.8；把它的小数部分扩大5倍，这个数是10.5．这个数原来是多少．
<result>6.9</result>