Not clear if you want B2 to be 24.15 or 24:15 format as 24.15 - the two are very different values.

For 24.15 as Decimal

=DOLLARFR(A2,60)
(DOLLARFR requires activation of Analysis ToolPak pre XL2007)

or

=INT(A2)+MOD(A2,1)*0.6
If you want B2 as hours but format as though decimal then:

=A2/24
format as [hh].mm