A salesman has a basic salary of Ksh 15,000 per month. Calculate his commission and total earnings for sales of Ksh 115,000 based on the tiered commission structure provided.

Sales amount: Ksh 50,000
Commission rate: 0%

$$\text{Commission}_1 = 50,000 \times 0\% = \text{Ksh } 0$$

This covers sales from Ksh 50,001 to Ksh 75,000.
Sales amount in this tier: Ksh $75,000 - 50,000 = 25,000$
Commission rate: $2.5\%$

$$\text{Commission}_2 = 25,000 \times \frac{2.5}{100} = 25,000 \times 0.025 = \text{Ksh } 625$$

This covers sales from Ksh 75,001 to Ksh 100,000.
Sales amount in this tier: Ksh $100,000 - 75,000 = 25,000$
Commission rate: $5\%$

$$\text{Commission}_3 = 25,000 \times \frac{5}{100} = 25,000 \times 0.05 = \text{Ksh } 1,250$$

Total sales: Ksh 115,000
Sales amount in this tier: Ksh $115,000 - 100,000 = 15,000$
Commission rate: $5\%$

$$\text{Commission}_4 = 15,000 \times \frac{5}{100} = 15,000 \times 0.05 = \text{Ksh } 750$$

$$\text{Total Commission} = \text{Commission}_1 + \text{Commission}_2 + \text{Commission}_3 + \text{Commission}_4$$ $$\text{Total Commission} = 0 + 625 + 1,250 + 750 = \text{Ksh } 2,625$$

The commission he earned that month is $\boxed{\text{Ksh } 2,625}$.

$$\text{Total Earning} = \text{Basic Salary} + \text{Total Commission}$$ $$\text{Total Earning} = 15,000 + 2,625 = \text{Ksh } 17,625$$

His total earning that month was $\boxed{\text{Ksh } 17,625}$.