If an athlete weighs 153lb and has 15.0% body fat by weight, how many pounds of fat does the athlete's body contain?

Break it down into things you know.
What's 10% of 153 lbs ?
SO you now know 10%, so what's 5% ?
Add the 2 terms together and you have 15%..

Percentages are fractions.

26% means 26/100
15% means 15/100

If you want to find 50% of $1000, you get (50/100)*1000 = $500.

The same method goes for your problem.