Calculation of Trade Discount:
To calculate the trade discount, we need to apply the given trade discount rate of 20% to the selling price of the salt. Let's say the selling price is SP.

Trade Discount = SP * Trade Discount Rate
Trade Discount = SP * 20/100
Trade Discount = SP/5

Calculation of Selling Price:
The selling price is given as 60% of the amount received by cheque. Let's say the amount received by cheque is X.

Selling Price = X * 60/100
Selling Price = 3X/5

Calculation of Cash Discount:
To calculate the cash discount, we need to apply the given cash discount rate of 5% to the selling price of the salt. Let's say the selling price after trade discount is SP1.

Cash Discount = SP1 * Cash Discount Rate
Cash Discount = SP1 * 5/100
Cash Discount = SP1/20

Calculation of Amount Received:
The amount received is given as 60% of the selling price after trade discount. Let's say the selling price after cash discount is SP2.

Amount Received = SP2 * 60/100
Amount Received = 3SP2/5

Calculation of Amount Spent:
The amount spent is the selling price after trade discount minus the cash discount.

Amount Spent = SP2 - Cash Discount
Amount Spent = SP2 - SP2/20
Amount Spent = 19SP2/20

Calculation of Amount Paid:
The amount paid is the amount received minus the cash discount.

Amount Paid = Amount Received - Cash Discount
Amount Paid = 3SP2/5 - SP2/20
Amount Paid = 12SP2/20
Amount Paid = 3SP2/5

Calculation of Trade Discount Amount:
The trade discount amount is the selling price minus the amount spent.

Trade Discount Amount = SP - Amount Spent
Trade Discount Amount = SP - 19SP2/20
Trade Discount Amount = SP/20 - 19SP2/20
Trade Discount Amount = SP * (1/20 - 19/20)

Calculation of Trade Discounted Price:
The trade discounted price is the selling price minus the trade discount amount.

Trade Discounted Price = SP - Trade Discount Amount
Trade Discounted Price = SP - SP * (1/20 - 19/20)
Trade Discounted Price = SP - SP * (-18/20)
Trade Discounted Price = SP * (1 + 18/20)
Trade Discounted Price = SP * 38/20
Trade Discounted Price = 19SP/10

Calculation of Trade Discounted Amount:
The trade discounted amount is the trade discounted price times the quantity sold.

Trade Discounted Amount = Trade Discounted Price * Quantity
Trade Discounted Amount = (19SP/10) * Quantity

Final Calculation:
The final amount paid by Kumar and son is the trade discounted amount minus the cash discount.

Final Amount Paid = Trade Discounted Amount - Cash Discount
Final Amount Paid = (19SP/10) * Quantity - SP/20

Therefore, the amount paid by Kumar and son for the salt is