Some white balls were put in a basket containing twelve red balls and sixteen black balls. If the probability of picking a white ball from the basket is 73, how many white balls were introduced?
Worked solution (try it first)
Let w white balls be added.
The total becomes 12+16+w=28+w.
So 28+ww=73.
Cross-multiply: 7w=84+3w.
Take 3w from both sides: 4w=84, so w=21, option C.