How to multiply a cell with currency format with a cell with time format

See attached Doc.

I am trying to work out a total cost. Every minute is charged at £1.56 (cell b4) so i am trying to multiply that with the figure in cell b2 which is 90mins and 6 seconds. So theoreticall it should be 1.56 * 90.1 (as 6 seconds is a tenth of a minute) How could i convert 90mins and 6 seconds into 90.1?!?

Any suggestions are very much appreciated.

Many thanks!!!