1. **Problem statement:** A sales representative has a monthly salary of 5000 TK. He earns a 5% commission on sales below 60000 TK, 8% commission on sales above 60000 TK, and a 1000 TK bonus if sales exceed 100000 TK. Given his total income is 10400 TK, find his total sales amount. 2. **Define variables:** Let total sales be $S$ TK. 3. **Analyze commission structure:** - If $S \leq 60000$, commission = $0.05S$. - If $60000 < S \leq 100000$, commission = $0.05 \times 60000 + 0.08 \times (S - 60000)$. - If $S > 100000$, commission = $0.05 \times 60000 + 0.08 \times (S - 60000)$ plus a 1000 TK bonus. 4. **Total income formula:** $$\text{Income} = \text{Salary} + \text{Commission} + \text{Bonus (if any)}$$ 5. **Given income:** 10400 TK. 6. **Check if bonus applies:** If $S > 100000$, income = $5000 + 0.05 \times 60000 + 0.08 \times (S - 60000) + 1000$. Calculate fixed part: $$0.05 \times 60000 = 3000$$ So, $$10400 = 5000 + 3000 + 0.08(S - 60000) + 1000$$ Simplify: $$10400 = 9000 + 0.08(S - 60000)$$ Subtract 9000: $$1400 = 0.08(S - 60000)$$ Divide both sides by 0.08: $$\frac{1400}{0.08} = S - 60000$$ $$17500 = S - 60000$$ Add 60000: $$S = 77500$$ 7. **Check if $S=77500$ satisfies bonus condition:** Since $77500 < 100000$, bonus should not apply. This contradicts our assumption. 8. **Try without bonus (i.e., $S \leq 100000$):** Income = $5000 + 0.05 \times 60000 + 0.08 \times (S - 60000)$ $$10400 = 5000 + 3000 + 0.08(S - 60000)$$ Simplify: $$10400 = 8000 + 0.08(S - 60000)$$ Subtract 8000: $$2400 = 0.08(S - 60000)$$ Divide: $$\frac{2400}{0.08} = S - 60000$$ $$30000 = S - 60000$$ Add 60000: $$S = 90000$$ 9. **Check bonus condition:** $90000 < 100000$, so no bonus applies. This is consistent. **Final answer:** The total sales amount is $\boxed{90000}$ TK.