A mill is equal to one thousandth of a dollar, so if the tax rate is 48 mills that means that the tax owed is $48 for each $1000 of assessed property value. So one can work backwards from tax paid to property value by dividing the tax paid by the mill rate and then multiplying by 1000. In this case, since Bill paid a tax of $5800, the assessed valuation of his property is equal to (5800/48)* 1000, which works out to $120,833.
