Let John's current age be $x$.

According to the problem, five years ago, John's age was half of what it will be in eight years. This gives the following equation:

$x - 5 = \frac{1}{2}(x + 8)$

Step 1: Eliminate the fraction

Multiply both sides of the equation by 2 to eliminate the fraction:

$2(x - 5) = x + 8$

Simplify:

$2x - 10 = x + 8$

Step 2: Solve for $x$

Subtract $x$ from both sides:

$2x - x - 10 = 8$

$x - 10 = 8$

Now, add 10 to both sides:

$x = 18$

Conclusion:

John is currently 18 years old.

Would you like further details or have any questions?

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Here are 5 related questions:

1. If John's age five years ago was a quarter of what it will be in ten years, how old is he now?
2. What will John's age be in 15 years?
3. If John's age in 3 years is twice his age 10 years ago, how old is he now?
4. How old was John two years ago if he will be 30 in five years?
5. If the sum of John's age and his brother's age is 40, and John is twice as old as his brother, how old are they?

Tip: When solving word problems, start by clearly identifying the unknowns and setting up an equation that translates the given information into a mathematical form.