Question

In 1975 1000 tons of coal was manufactured. It is 10% greater in 1976 and 5% less in 77. What is the final about of coal in 1977

Answer 1

In 1975 it was 1000tons, ok, well, in 1976 it went up by 10%, how much is a 10% increase?

$\bf \begin{array}{ccllll} coal&\%\\ \text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\ 1000&100\\ x&100+10 \end{array}\implies \cfrac{1000}{x}=\cfrac{100}{100+10}\implies \cfrac{1000\cdot 110}{100}=x \\\\\\ 1100=x$

well, in 1976 it was 1100 tons then, and in 1977 it went down by 5% of 1100, well, if we take 1100 to be the 100%, then what is a 5% decrease?

$\bf \begin{array}{ccllll} coal&\%\\ \text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\ 1100&100\\ x&100-5 \end{array}\implies \cfrac{1100}{x}=\cfrac{100}{100-5}\implies \cfrac{1100\cdot 95}{100}=x$