15
1) Since the question is "How many more people said their favorite color is white compared to those who said their favorite color is black? "
Let's turn those percentages into absolute numbers, rewriting those percentages dividing each figure by 100.
250 x 0.04 =10 people chose silver
250 x 0.32 =80 blue
250 x 0.02 =5 green
250 x 0.2 = 50 red
250 x 0.24= 60 white
250 x 0.18= 45 black
2) Now let's compare those people who chose white over black and then
60 people chose white
45 people chose black
60-45 = 15 people
3) Hence, 15 people more chose black as their favorite color.