Cost of n Shirts with a Discount

The problem: a shop sells shirts. Ask how many the customer is buying, the price of one shirt, and the discount percent. Print the bill: full amount, discount given, and the amount to pay.

1Plan it first

Three inputs this time, and three lines of working. Writing the working out as separate steps means each one can be printed and checked:

Input

what we ask the user for

number of shirts, n
price of one shirt, price
discount percent, d

Process

what we work out

total = n * price
discount = total * d / 100
payable = total - discount

Output

what we show

total before discount
discount amount
amount to pay

2The program

shirt_bill.py

# bill for n shirts with a discount

n = int(input('How many shirts? '))
price = float(input('Price of one shirt: '))
d = float(input('Discount percent: '))

total = n * price
discount = total * d / 100
payable = total - discount

print('Total before discount =', total)
print('Discount =', discount)
print('Amount to pay =', payable)

Output

How many shirts? 3
Price of one shirt: 499
Discount percent: 10
Total before discount = 1497.0
Discount = 149.7
Amount to pay = 1347.3

int() for the count — nobody buys 2.5 shirts — float() for the money and the percent. Run it and change the numbers:

shirt_bill.py

n = int(input('How many shirts? '))
price = float(input('Price of one shirt: '))
d = float(input('Discount percent: '))

total = n * price
discount = total * d / 100
payable = total - discount

print('Total before discount =', total)
print('Discount =', discount)
print('Amount to pay =', payable)

n = int(input('How many shirts? '))
price = float(input('Price of one shirt: '))
d = float(input('Discount percent: '))

total = n * price
discount = total * d / 100
payable = total - discount

print('Total before discount =', total)
print('Discount =', discount)
print('Amount to pay =', payable)

3What a percent actually is

“10 percent” means 10 out of every 100. There is no percent sign in the maths — you divide by 100:

percent.py

total = 1497.0
d = 10

discount = total * d / 100

print(discount)

Output

149.7

% is not percent in Python

Python does have a % operator, but it is modulus — the remainder after division. total * d % 100 would give you something meaningless. To take a percentage, always * d / 100.

There is a shorter way to reach the same answer. Paying after a 10% discount is paying 90% of the bill, so you can skip the middle step:

shorter.py

total = 1497.0
d = 10

payable = total * (100 - d) / 100

print(payable)

Output

1347.3

Same answer. The longer version is still the better program here, because the customer wants to see how much they saved — and a bill that shows its working is easier to trust.

4Now you try

Add GST of 5% to the payable amount and print the final total.
Print how much is saved per shirt (discount / n).
Run it with a discount of 0 and of 100. Does the bill still make sense?

your_turn.py

n = int(input('How many shirts? '))
price = float(input('Price of one shirt: '))
d = float(input('Discount percent: '))

total = n * price
discount = total * d / 100
payable = total - discount

print('Total before discount =', total)
print('Discount =', discount)
print('Amount to pay =', payable)

n = int(input('How many shirts? '))
price = float(input('Price of one shirt: '))
d = float(input('Discount percent: '))

total = n * price
discount = total * d / 100
payable = total - discount

print('Total before discount =', total)
print('Discount =', discount)
print('Amount to pay =', payable)

5Recap

Key Takeaway

A percentage in code is amount * percent / 100 — never the % operator, which means remainder. Use int() for counts and float() for money. Breaking the bill into total, discount and payable lets you print each line and spot a wrong one instantly.

Quick Check

Which expression gives 10% of total?

Quick Check

3 shirts at ₹499 with 10% off. What is the amount to pay?

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