Fractions, percentages and multiplying by powers of ten

Adding fractions needs a common denominator

You cannot add fifths to tenths any more than you can add metres to seconds. Rewrite one of them: $\tfrac25=\tfrac4{10}$, so $\tfrac25+\tfrac3{10}=\tfrac7{10}$.

Percentages

Per cent means 'out of a hundred'. Work through the $10\%$ landmark: $10\%$ of $250$ is $25$, so $30\%$ is $75$ and $5\%$ is $12.5$.

Times and divide by ten

Multiplying by $10$ moves every digit one column left; dividing moves it right. The decimal point does not move — the digits do. Thinking of it that way stops the classic 'just add a zero' error with decimals.