Plugging In: Basic Technique

If in $$k+1$$ years from now, John will be $$m$$ years old, then how old was John $$t-1$$ years ago?

Now go to the answer choices to eliminate any that do not match your goal value. **Plug In** all variables and eliminate any that do not equal 2. >A) $$m-k-t = 10-3-5 = \color{blue}{2}$$. This doesn't automatically mean that A is the answer, since we need to check all five choices. But it does mean we cannot eliminate A yet. >B) $$m+k-t+2 = 10+3-5+2 = \color{red}{10}$$. This is not equal to 2. __POE__ this answer choice and move on. >C) $$m+k-t+1 = 10+3-5+1 = \color{red}{9}$$ . This is not equal to 2. __POE__ this answer choice and move on. >D) $$k-m-t+2 = 3-10-5+2$$. That's going to be negative, and not equal to 2. __POE__ this answer choice and move on. >E) $$k+t-m = 3+5-10 = 8-10= \color{red}{-2}$$. This is not equal to 2. __POE__ this answer choice. Answer choice A is the only answer choice that is equal to 2 for $$k=3$$, $$m=10$$, and $$t=5$$ and thus must be the right answer choice. How did you solve this?

Incorrect. [[snippet]] If you plug in $$k=3$$, $$m=10$$, and $$t=5$$, then the goal you get is 2. However, for this answer choice is equal to >$$m+k-t+2 = 10+3-5+2 = 10$$ This is not equal to 2. __POE__ this answer choice and move on.

Incorrect. [[snippet]] If you plug in $$k=3$$, $$m=10$$, and $$t=5$$, then the goal you get is 2. However, for this answer choice is equal to >$$m+k-t+1 = 10+3-5+1 = 9$$ This is not equal to 2. __POE__ this answer choice and move on.

Incorrect. [[snippet]] If you plug in $$k=3$$, $$m=10$$, and $$t=5$$, then the goal you get is 2. However, for this answer choice is equal to >$$k-m-t+2 = 3-10-5+2 $$ That's going to be negative and not equal to 2. __POE__ this answer choice and move on.

Incorrect. [[snippet]] If you plug in $$k=3$$, $$m=10$$, and $$t=5$$, then the goal you get is 2. However, for this answer choice is equal to >$$k+t-m = 3+5-10 = 8-10=-2$$ This is not equal to 2. __POE__ this answer choice.

Be careful! Questions will quickly become much more difficult and if you use algebra you will: 1. Greatly increase your chance of making mistakes. 2. Waste precious time. So from now on, try to practice __Plugging In__—even for easy questions. That way you'll sharpen your __Plugging In__ skills for the really tough questions that will get you a high score.

Very good.

Correct. [[snippet]] Just pick some values for the variables. Try $$k=3$$, $$m=10$$, and $$t=5$$. Then rewrite the question with the values instead: *"If in 4 years from now, John will be 10 years old, then how old was John 4 years ago?"* >*"If in 4 years from now, John will be 10 years old"* This means he is currently 6 years old. >*"how old was John 4 years ago?"* If he is 6 now, then he was 2 years old 4 years ago. Since the answer to the question is 2, then 2 is your **goal value**.

$$m-k-t$$

$$m+k-t+2$$

$$m+k-t+1$$

$$k-m-t+2$$

$$k+t-m$$

I solved it by __Plugging In__.

I solved it using algebra.

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