To solve this expression, we need to convert the mixed numbers to improper fractions.

13 3/5 = (5 * 13 + 3) / 5 = 68/5
4 3/4 = (4 * 4 + 3) / 4 = 19/4
8 3/5 = (5 * 8 + 3) / 5 = 43/5

Now we can substitute these values back into the expression:

b = (68/5 + 19/4) - 43/5

To add the fractions, we need a common denominator. The least common multiple of 5 and 4 is 20.

b = (68/5 * 4/4 + 19/4 * 5/5) - 43/5

b = (272/20 + 95/20) - 43/5

Now that the fractions have a common denominator, we can combine them:

b = (272 + 95) / 20 - 43/5

b = 367/20 - 43/5

To subtract the fractions, we need a common denominator. The least common multiple of 20 and 5 is 20.

b = (367/20 - 43/5 * 4/4)

b = (367/20 - 172/20)

Now that the fractions have a common denominator, we can subtract them:

b = (367 - 172) / 20

b = 195/20

The fraction 195/20 can be simplified by dividing both the numerator and denominator by their greatest common divisor, which is 5:

b = (195/5) / (20/5)

b = 39/4

Therefore, b = 39/4.