Explanation

The 50% spent on rent, utilities, and insurance and the 20% spent on food are both percents of the total, so sum the percents: 50% + 20% = 70%. After these expenditures, Aloysius has 30% left.

He then spends 30% of the remaining 30% on video games. 30% of 30% = 0.30 × 0.30 = 0.09, or 9% of the total, so 30% – 9% = 21% of his income remains.


Alternatively, use smart numbers. If Aloysius’s income is $100, he would spend $50 on rent, utilities, and insurance, and $20 on food, for a total of $70. Of his remaining $30, he would spend 30%, or $9, on video games, leaving $21, or 21%, of the original amount.