What is the answer? and why is that the answer?

Since you have a ratio of how many galleons to galleys were in the Spanish Armada, the information you would need to find the total amount of galleons and galleys would be how many the Armada had of one of the two kinds.

For example, if we know that the Armada had 100 galleons, then we can use the ratio of 5 galleons : 1 galley to find out how many galleys it had. Then we can add the two numbers to find out how many ships total.

So, you need either the amount of galleons, or the amount of galleys in order to figure out the total amount of galleons and galleys in the Spanish Armada.