His new apartment will cost him "10% more" than what he is paying currently.
100% = current rent
10% = the extra amount he will pay for his new apartment
100% + 10% = new rent
100% + 10% = 110%
Now convert the percent into a decimal by moving the % sign two places to the left and make the % sign into a decimal point.
110% = 1.10
500 * 1.10 = 550
So, he will pay a rent of $550 for his new apartment.
