Answer
$10$ years.
Work Step by Step
A difference is the result of subtraction
$\left.\begin{array}{ccc}
\text{(effect of working puzzles)} & \text{minus} & \text{ (effect of eating red meat)}\\
5 & - & (-5)
\end{array}\right.$
$ 5-(-5)=\qquad$subtract $-5$ by adding $5$, its additive opposite
$=5+5=10$
