Two fifths of the instruments in the marching band are brass. one-eighth of the brass instruments are tubas. what fraction of the band is tubas? "

2/5 are brass, 1/8 of 2/5 are tubas
When you have the word "of" you're going to use multiplication. 
40% is brass (you know this by dividing 2 by 5 for the 2/5ths ratio), so you'll want to know what 1/8 of that is. 
40*(1/8) = 5 (this is already a percent)
You can also take 1/8 and multiply it by 2/5. You end up with the same thing, but it would be written as .05. To make this into a percent you multiply it by 100 to get 5%
Your final answer is 5% of the band are tubas.