title: How to sum a number of rows based on a value in a different cell?
body: <p>I am trying to make a formula that will calculate how much of my mortgage I have paid in principal given the number of months I specify.  I have a worksheet that has my Loan Amortization Schedule, and on a second worksheet I am trying to figure out how much of the principal I have paid down in 12 months, 24 months, 47 months, etc.  </p>
  
  <p>I would like to know how to calculate this.  In one cell I'll enter the number of months (12, 24, 47, etc.) and then in a cell directly below it I will put the formula that will return all of the principal payments in a particular column on my Loan Amortization Schedule worksheet given whatever value I had put for the number of months (again 12, 24, 47, etc.).</p>
  
  <p>Can someone plesae help?</p>