Account Balance After Rounded Purchase

Account Balance After Rounded Purchase - Problem

Math Easy

Initially, you have a bank account balance of 100 dollars.

You are given an integer purchaseAmount representing the amount you will spend on a purchase in dollars, in other words, its price.

When making the purchase, first the purchaseAmount is rounded to the nearest multiple of 10. Let us call this value roundedAmount. Then, roundedAmount dollars are removed from your bank account.

Return an integer denoting your final bank account balance after this purchase.

Notes:

0 is considered to be a multiple of 10 in this problem.
When rounding, 5 is rounded upward (5 is rounded to 10, 15 is rounded to 20, 25 to 30, and so on).

Input & Output

Example 1 — Basic Rounding Down

$ Input: purchaseAmount = 9

› Output: 90

💡 Note: 9 is closer to 10 than to 0, so it rounds to 10. Final balance: 100 - 10 = 90

Example 2 — Rounding Up with Tie

$ Input: purchaseAmount = 15

› Output: 80

💡 Note: 15 is exactly between 10 and 20, so it rounds up to 20. Final balance: 100 - 20 = 80

Example 3 — No Rounding Needed

$ Input: purchaseAmount = 10

› Output: 90

💡 Note: 10 is already a multiple of 10, no rounding needed. Final balance: 100 - 10 = 90

Constraints

0 ≤ purchaseAmount ≤ 100

Visualization

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Asked in

G Google 12 a Amazon 8

Brief HTML summary: The key insight is to use mathematical rounding by adding 5 first, then dividing by 10 and multiplying back. This automatically handles the upward rounding rule for ties. Best approach is the mathematical formula. Time: O(1), Space: O(1)

Common Approaches

✓ Brute Force Check

⏱️ Time: O(1) Space: O(1)

Find the lower and upper multiples of 10 around the purchase amount, then determine which is closer. Handle the tie-breaking rule for numbers ending in 5.

Mathematical Rounding

⏱️ Time: O(1) Space: O(1)

Add 5 to the purchase amount, then divide by 10 and multiply back by 10. This automatically handles the rounding rule where 5s round upward.

Brute Force Check — Algorithm Steps

Step 1: Find the lower multiple of 10 (floor division)
Step 2: Find the upper multiple of 10
Step 3: Compare distances and apply tie-breaking rule
Step 4: Subtract from 100

Visualization

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Step-by-Step Walkthrough

Find Boundaries

Calculate lower and upper multiples of 10

Compare Distances

Determine which multiple is closer

Apply Rule

Round up in case of tie, subtract from 100

Code -

solution.c — C

#include <stdio.h>

int solution(int purchaseAmount) {
    int lower = (purchaseAmount / 10) * 10;
    int upper = lower + 10;
    
    if (purchaseAmount - lower < upper - purchaseAmount) {
        return 100 - lower;
    } else if (purchaseAmount - lower > upper - purchaseAmount) {
        return 100 - upper;
    } else {
        return 100 - upper;  // tie case - round up
    }
}

int main() {
    int purchaseAmount;
    scanf("%d", &purchaseAmount);
    
    int result = solution(purchaseAmount);
    printf("%d\n", result);
    
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(1)

Fixed number of mathematical operations regardless of input size

✓ Linear Growth

Space Complexity

O(1)

Only using a few variables to store intermediate calculations

✓ Linear Space

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Input & Output

Constraints

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Related Problems

Common Approaches

Brute Force Check — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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