Some rate questions asks for combined rate. For these, first determine the rate, and then add them together.
Example: "Lionel would take 8 days to build a wall. For the same wall, Nick would take 6 days. How many days would be needed to build the same wall if both of them worked together?"
Lionel rate = 1/8 wall per day
Nick rate = 1/6 wall per day
Combined rate = 1/8 + 1/6 = 3/24 + 4/24 = 7/24 of a wall per day.
For one wall, time taken = 1 / (7/24) = 24/7 days.
To quickly check, notice that 2 of Lionel will take 4 days, and 2 of Nick will take 3 days, so Lionel and Nick will take between 3 and 4 days.