Last year the first $ 30,000 of each Country X resident's annual taxable income was taxed at the rate of 15 percent, and taxable income in excess of $ 30,000 was taxed at a rate of 27 percent. If Mr. Smith was a resident last year, was all of his taxable income taxed at the 15 percent rate?Basically the question asks whether Mr. Smith's income was less than or equal to $30,000. Also notice that for $30,000 tax is $30,000*0.13=$4,500.
(1) Last year Mr. Smith's tax, based on his taxable income, was $ 3,750. Since the tax was less than $4,500 then the income was less than $30,000. Sufficient.
(2) If Mr. Smith's taxable income had been twice as much as it was, his tax would have been $ 9,900. From $9,900, $4,500 was paid on $30,000 and the rest $5,400 were paid on an additional income (income over $30,000) at a rate of 27%. Additional income would be: x*0.27=$5,400 --> x=$20,000. So, if Mr. Smith's taxable income had been twice as much it would be $30,000+$20,000=$50,000, so his actual income is $50,000/2=$25,000. Sufficient.
Answer: D.
Hope it's clear.