Solve the Equation: Finding the Missing Number in -25-?=100

Q: Why do I add the question mark to both sides instead of subtracting 100 first?

Adding the unknown to both sides eliminates the subtraction on the left side immediately! This gives you $ -25 = 100 + ? $, which is much easier to solve than working with negative operations.

Q: How do I know if the answer should be positive or negative?

Look at the original equation structure! Since we have $ -25 - ? = 100 $, we need a negative number because subtracting a negative makes the result larger (more positive).

Q: Can I solve this by moving terms around differently?

Yes! You could add 25 to both sides first: $ -? = 125 $, then multiply by -1. However, the method shown is often clearer for beginners.

Q: What if I get confused about the signs?

Remember: Subtracting a negative number is the same as adding! So $ -25 - (-125) = -25 + 125 = 100 $. Practice this concept with smaller numbers first.

Solving Equations with Missing Subtrahends

Solve the following problem:

$-25-?=100$

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations

00:00 Find the unknown

00:08 Isolate the unknown

00:26 Reduce what's possible

00:39 Isolate the unknown

00:56 Calculate each operation separately

01:10 And this is the solution to the question

Step-by-step written solution

Follow each step carefully to understand the complete solution

Understand the problem

Solve the following problem:

$-25-?=100$

Step-by-step solution

We'll add ?+ to both sides to zero out the left side:

$-25-?+?=100+?$

$-?+?=0$

Therefore, we obtain:

$-25-0=100+\text{?}$

$-25=100+\text{?}$

We'll add minus 100 to both sides to zero out the right side:

$-25-100=100-100+\text{?}$

We should obtain the following:

$-125=0+\text{?}$

$-125=\text{?}$

Final Answer

$-125$

Key Points to Remember

Essential concepts to master this topic

Isolation Rule: Add the unknown variable to both sides first
Technique: Transform $-25-?=100$ to $-25=100+?$
Check: Substitute back: $-25-(-125)=-25+125=100$ ✓

Common Mistakes

Avoid these frequent errors

Subtracting instead of adding to isolate the unknown
Don't subtract 100 from both sides first = $-125-?=0$ which makes it harder! This creates unnecessary complexity and confusion. Always add the unknown variable to both sides first to simplify the equation structure.

Practice Quiz

Test your knowledge with interactive questions

a is negative number.

b is negative number.

What is the sum of a+b?

FAQ

Everything you need to know about this question

Why do I add the question mark to both sides instead of subtracting 100 first?

Adding the unknown to both sides eliminates the subtraction on the left side immediately! This gives you $-25 = 100 + ?$ , which is much easier to solve than working with negative operations.

How do I know if the answer should be positive or negative?

Look at the original equation structure! Since we have $-25 - ? = 100$ , we need a negative number because subtracting a negative makes the result larger (more positive).

Can I solve this by moving terms around differently?

Yes! You could add 25 to both sides first: $-? = 125$ , then multiply by -1. However, the method shown is often clearer for beginners.

What if I get confused about the signs?

Remember: Subtracting a negative number is the same as adding! So $-25 - (-125) = -25 + 125 = 100$ . Practice this concept with smaller numbers first.

How can I check my work quickly?

Substitute your answer back into the original equation. Replace the ? with your answer and calculate: $-25 - (-125)$ . If you get 100, you're correct!

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