I need a formula to calculate averages, from averages.

In column “A” I have different times in minutes and in column “B” I have average rainfall for that time.

Column “A” consists of “5min”, “6min”, “10min”, “20min”

Column “B” consists of “253mm”,”236mm”, “189mm”,”134mm”

What I require is,

If a user inputs a value for rainfall into cell “D1”, say “248mm” it can calculate the average time based on the above data.

Thanks