Simple Interest Program Python में — code और dry run

Problem statement

Approach 1: simple loop

Apply the formula directly. The division by 100 exists because R is a percentage, not a fraction — 7.5 means 7.5 per hundred.

p = 5000        # principal (rupees)
r = 7.5         # rate per year (percent)
t = 2           # time (years)

si = p * r * t / 100
print("Interest:", si)
print("Total:", p + si)
# Output:
# Interest: 750.0
# Total: 5750.0

Multiplication and division have equal precedence and run left to right, so p * r * t / 100 needs no brackets.

Approach 2: reusable function

Wrap it in a function so the inputs and the formula are separated — then the same three lines serve savings accounts, loans, and exam questions alike.

def simple_interest(p, r, t):
    return p * r * t / 100

def total_amount(p, r, t):
    return p + simple_interest(p, r, t)

print(simple_interest(5000, 7.5, 2))    # 750.0
print(total_amount(5000, 7.5, 2))       # 5750.0
print(simple_interest(12000, 10, 1))    # 1200.0

One function, one job — and total_amount builds on it instead of repeating the formula.

Approach 3: Pythonic alternative

Real programs read inputs and format outputs. Convert at the boundary with float(), and present money with f-string formatting to two decimals.

p = float(input("Principal: "))
r = float(input("Rate % per year: "))
t = float(input("Years: "))

si = p * r * t / 100
print(f"Interest = {si:.2f}")
print(f"Total payable = {p + si:.2f}")
# For inputs 5000, 7.5, 2:
# Interest = 750.00
# Total payable = 5750.00

{si:.2f} prints exactly two decimals — 750.00 instead of 750.0 — which is what money deserves.

Dry run

Common errors

• / 100 भूल जाना — जवाब सौ गुना बड़ा आ जाता है।
• input() को float() में convert न करना।
• Simple और compound interest mix करना — compound में p * (1 + r/100) ** t होता है।
• जोड़ने से पहले interest को round करना — round सिर्फ दिखाने के लिए करो।
• Rate को fraction (0.075) लेकर भी 100 से भाग देना — एक convention चुनो और उसी पर रहो।

Practice on PyDebug

Concept समझने के बाद इन free problems को browser में solve करें:

• Range boundary practice
• Operator precedence practice
• Variables and assignment practice