Explanation:
solving for x means that we end up with something like
x = ...
or
... = x
remember, whatever e do, we must do on both sides of the equation.
z = 4(n + p)/(5x)
what disturbs us most is that there is a division (or fraction) AND that x is in the denominator (bottom).
so, let's multiply both sides by 5x :
5x×z = 5x×4(n + p)/5x = 4(n + p)
now, what is blocking us from our target form is that x has a factor of 5z.
so, let's divide both sides by 5z :
5xz / 5z = 4(n + p) / 5z
x = 4(n + p)/(5z)
done.