When matter and antimatter collide, they disappear. When a number and its multiplicative inverse collide, they also disappear. But this is algebra, not particle physics. You can create the multiplicative inverse, or reciprocal, of a number by writing a fraction with the number in the denominator and 1 in the numerator. The multiplicative inverse of 3, for example, is 1/3. Multiplying 3 by its inverse, therefore, leaves you with 1. This property makes solving for x in a numerator easy.
Write the equation you wish to solve for x. For example, your write "x/3 = 7."
Note the number that forms the denominator of the fraction of which x is the numerator. In the example, 3 is the denominator of the fraction.
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Multiply each term in the equation by the number in the denominator. For example, 3 * (x/3) = 3 * 7
Remember that multiplying any fraction by the term in its denominator eliminates that denominator. For example, 3 * (x/3) = x.
Simply the left side of your equation using this knowledge. In the example, x = 3 * 7.
Complete the multiplication on the right side of your equation: x = 21.