I know that 0.6 of an hour equals 40 minutes but how do you work this out?

.6 of an hour is not 40 minutes, it is 36 minutes. This is worked out by multiplying the decimal and the amount of minutes in an hour.

.6 x 60 = 36 minutes

Example

.5 x 60 = 30 minutes, half of an hour equals half of an hour.