how many sharps in an a major scale
All you need to do it find the last sharp and go up one semitone. This will give you the major scale associated with that key. E.g. If there are 3 sharps in a key signature (F, C, G) look at the last sharp (G) and go up one semitone (A). So 3 sharps is the key of A Major. The second shortcut isn't for all of the sharp keys, but it works for some