20 identical articles are sold at ₹ 350 each with GST = 12%. 10 other type of articles are sold at ₹ 500 each with GST = 18%. Total amount of bill is :

1. ₹ 12000

2. ₹ 13440

3. ₹ 13740

4. ₹ 14160

For ₹ 350 articles,

Total amount = No. of articles × ₹ 350

= 20 × ₹ 350 = ₹ 7000.

Given, GST = 12%

GST = $\dfrac{12}{100} \times 7000$ = ₹ 840.

Amount (including GST) = ₹ 7000 + ₹ 840 = ₹ 7840.

For ₹ 500 articles,

Total amount = No. of articles × ₹ 500

= 10 × ₹ 500 = ₹ 5000.

Given, GST = 18%

GST = $\dfrac{18}{100} \times 5000$ = ₹ 900.

Amount (including GST) = ₹ 5000 + ₹ 900 = ₹ 5900.

Total amount = ₹ 7840 + ₹ 5900 = ₹ 13740.

Hence, Option 3 is the correct option.