It isn't easy. You just have to go slowly and keep your wits about you.
-- Add the seconds. If the sum is more than 60, then take 60 off, write what's left
as seconds, and carry 1 minute next door to the minutes.
-- Add the minutes. If the sum is more than 60, then take 60 off, write what's left
as minutes, and carry 1 degree next door to the degrees.
-- Add the degrees.
You can use exactly the same procedure to add hours, minutes, and seconds
of time.
