Calculation from time stamps through to amount due

[Brahmanatha Swami]

I have three fixed data columns in a table used to log developer time and payment due

start_time end_time

I have this trigger which works:

set new.elapsed=(TIMESTAMPDIFF(MINUTE , new.start_time , new.end_time ) / 60.00);
# the total time will be entered into the column "elapsed"
# now I want to multiply the rate e.g. $40.00 per hour by the elapsed time
# to get the amount do for each row.
# but the following throws and error.

set new.elapsed=(TIMESTAMPDIFF(MINUTE , new.start_time , new.end_time ) / 60.00;
set new.amount_due=(elapsed * rate)

What am I doing wrong?