You would only need 2 global variables, a level score and a total score.
When you finish level 1, level score = 50, total score = 50
finish level 2, level score = 100, total score = total score + level score, so 100 + 50 = 150
finish level 3, level score = 150, total score = total score + level score, so 150 + 150 = 300
You just add the level score to total at the end of each level.