Converting seconds (raw number) to minutes (mm:ss format)

Perhaps this is a stupid question, but my brain seems to be failing me right now.

Cell A1 contains this number representing the total number of seconds to complete a task: 90

I need cell A2 to convert that number into the minutes:seconds format: 1:30

AND, convert the other direction.

Cell B1 contains this number representing minutes and seconds to complete a task: 1.30

I need cell B2 to convert that number into seconds: 90

Am I missing some simple computations or are there some complex formulas that will make these conversions?

Any help will be greatly appreciated. If this is some simple thing I'm missing, I'll accept the humiliation to get this figured out. Thanks