a) To add the fractions, we need to find a common denominator. The least common multiple of 5, 3, and 2 is 30.

So, we rewrite the fractions with the common denominator of 30:

4/5 = 24/30
2/3 = 20/30
4/5 = 24/30

Now, we can add the fractions:

24/30 + 20/30 - 24/30 = (24 + 20 - 24) / 30 = 20 / 30 = 2/3

Therefore, the result of the expression 4/5 + 2/3 - 4/5 is 2/3.

b) To subtract the fractions, we need to find a common denominator. The least common multiple of 2, 4, and 3 is 12.

So, we rewrite the fractions with the common denominator of 12:

1/2 = 6/12
-3/4 = -9/12
-1/2 = -6/12
2/3 = 8/12
-1/2 = -6/12

Now, we can subtract the fractions:

6/12 - 9/12 - 6/12 - 8/12 - (-6/12) = (6 - 9 - 6 - 8 + 6) / 12 = (-11) / 12 = -11/12

Therefore, the result of the expression 1/2 - 3/4 - 1/2 - 2/3 - 1/2 is -11/12.