L42.54 is the same as 42 ' 24".

There are 60 ( ' ) minutes in 1 degree, and 60 ( " ) seconds in 1 minute.

now, on a Latitude of 42.54°, there are 42° clearly, how much is the 0.54 anyway?

well, we know there are 60 minutes in 1 degree, therefore 0.54 * 60 is how many minutes in 0.54, or 32.4'.  So a whole 32' and 0.4 of a minute.

how many seconds in 0.4 minutes?   well, there are 60 seconds in 1 minute, thus 0.4* 60, or 24.

L42.54  is then 42° 32' 24".