Sometimes you can easily find the value of a symbol through Guess and Check. In Guess and Check, you guess at what you believe is the answer, then plug it into the equation to see if you are correct. 
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Arithmetic Let's do the following problem with standard arithematic. A puppy weighed 15 pounds when Jeff bought it. It gained 3 pounds. How much does the puppy weigh now? Let the new weight of the puppy be x. We determine the answer by adding 3 to 15, as in the following math sentence: 15 + 3 = x Solving the math equation we get: 15 + 3 = 18 Jeff's puppy now weighs 18 lbs. 

Algebra (guessing) Now let's try a word problem using Guess and Check. Jane's puppy gained 5 lbs, and now weighs 22 lbs. What did the puppy weigh when she bought it? Let the weight of the puppy be x, when he bought it. So, the math sentence is: x + 5 = 22 Let's guess the answer is 18. Substituting 18 for x makes the math sentence: 18(?) + 5 = 22, which is not correct since 18 + 5 = 23 So, let's guess 17. 17 + 5 = 22, which is true. So, x = 17 The puppy weighed 17 pounds when Jane bought the puppy. This was a simple problem, and clearly you could have solved it using subtraction (22  5 = 17). However, sometimes if you don't know what to do, Guess and Check is a potential way to find the answer. 

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