Question

Andy estimated that he would need 78 ft of a lumber for a tree house project. He later found that the actual amount of lumber needed was 67 ft. What was the percent error of Andy's estimation
A. 16.42%
B. 11℅
C. 14.10℅
D. 85.90℅

Answer 1

If we take 78 as the 100%, what's 11 of it?
well  $\bf \begin{array}{ccllll} amount&\%\\ \textendash\textendash\textendash\textendash\textendash\textendash&\textendash\textendash\textendash\textendash\textendash\textendash\\ 78&100\\ 11&x \end{array}\implies \cfrac{78}{11}=\cfrac{100}{x}$

solve for "x"