Given:
Rate of GST = 18%
Since the mechanic and all the three customers are in Mumbai, it is a case of intrastate transaction.
Let us first find the repairing cost of each customer after discount.
For Customer I:
Discount = 10% of ₹ 3,000 = ₹ $\Big(\frac{10}{100} \times 3000\Big)$ = ₹ 300.
Cost after discount = ₹ (3,000 - 300) = ₹ 2,700.
For Customer II:
Discount = 5% of ₹ 4,000 = ₹ $\Big(\frac{5}{100} \times 4000\Big)$ = ₹ 200.
Cost after discount = ₹ (4,000 - 200) = ₹ 3,800.
For Customer III:
Discount = 8% of ₹ 2,500 = ₹ $\Big(\frac{8}{100} \times 2500\Big)$ = ₹ 200.
Cost after discount = ₹ (2,500 - 200) = ₹ 2,300.
Total cost after discount = ₹ (2,700 + 3,800 + 2,300) = ₹ 8,800.
(i) Since it is the case of intrastate transaction, so IGST = 0.
Hence, IGST = 0.
(ii) GST rate = 18%
⇒ CGST rate = 9% and SGST rate = 9%.
CGST = 9% of ₹ 8,800
= ₹ $\Big(\frac{9}{100} \times 8800\Big)$
= ₹ 792.
Hence, CGST = ₹ 792.
(iii) GST rate = 18%
SGST = 9% of ₹ 8,800
= ₹ $\Big(\frac{9}{100} \times 8800\Big)$
= ₹ 792.
Hence, SGST = ₹ 792.
(iv) For Customer I:
CGST = 9% of ₹ 2,700 = ₹ 243 and SGST = 9% of ₹ 2,700 = ₹ 243.
Amount of the bill = ₹ (2,700 + 243 + 243) = ₹ 3,186.
For Customer II:
CGST = 9% of ₹ 3,800 = ₹ 342 and SGST = 9% of ₹ 3,800 = ₹ 342.
Amount of the bill = ₹ (3,800 + 342 + 342) = ₹ 4,484.
For Customer III:
CGST = 9% of ₹ 2,300 = ₹ 207 and SGST = 9% of ₹ 2,300 = ₹ 207.
Amount of the bill = ₹ (2,300 + 207 + 207) = ₹ 2,714.
Hence, the total amount of bill paid is ₹ 3,186 by Customer I, ₹ 4,484 by Customer II and ₹ 2,714 by Customer III.