To determine the least number that needs to be added to $78989$ to make it divisible by 4, we follow these steps:

Step 1: Check divisibility by 4

A number is divisible by 4 if the number formed by its last two digits is divisible by 4.

The last two digits of $78989$ are $89$.

Step 2: Find the remainder

Divide $89$ by $4$: $89 \div 4 = 22 \, \text{remainder} \, 1$

This means $89$ leaves a remainder of $1$ when divided by $4$. Therefore, $78989$ is not divisible by $4$.

Step 3: Calculate the required addition

To make $89$ (and thus $78989$) divisible by $4$, we need to add the complement of the remainder to $89$: $4 - 1 = 3$

So, the least number that can be added to $78989$ is $3$.

Final Answer:

The least number that needs to be added is 3.

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Related Questions:

1. How is divisibility by other numbers (like 3, 6, or 9) determined?
2. What are some practical applications of divisibility rules?
3. How can we check divisibility for larger numbers without performing division?
4. How do divisibility rules differ in modular arithmetic?
5. Can divisibility rules be extended to decimal and fractional numbers?

Tip:

Always verify divisibility by focusing on small groups of digits or specific rules, as they can simplify complex calculations.