Answer:
15p
Explanation:
Let's interpret the question bit by bit
Salvador wrote 15 letters to friends each month
=> 15 letters = 1 month
For p months in a row
let the total number of letters for p months be x
=> x letters = p months
Bringing the two statements together gives;
15 letters = 1 month
x letters = p months
Now, let's solve for x by cross-multiplying the above
x letters * 1 month = 15 letters * p months
x =

x = 15p letters
Therefore, Salvador wrote a total of 15p letters for the p months.