Evaluate an equation by substitution

The video tutorial explains how to solve for the variable b in the equation a + b + c = 180, where b = 3a and c = 5a. The teacher demonstrates using substitution to replace b and c with expressions in terms of a, simplifying the equation to a single variable. By combining like terms, the equation becomes 9a = 180, which is solved to find a = 20. Finally, the value of b is calculated as 3 times a, resulting in b = 60.

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