Replaceing a cell with its added value to another cell

I am trying to add daily and month to date columns. I want the month to date to add the amount in its own column to the value I type in the daily column and change the month to date to account for the increase. Ex. I have $20.00 in the MTD column and I put $10.00 in the daily column, I want the MTD to change to $30.00. Can you help me?