When magnesium is put into an acid
it will react and produce hydrogen gas (see more information).
The more concentrated the acid, the faster the magnesium will react
and you will see more bubbles of hydrogen being produced.

A strong acid and a weak acid of the same concentration
will react at different rates with the same metal.

You can see the difference between
a strong acid and a weak acid of the same concentration
by looking at the reaction with magnesium.
The strong acid will react faster and you will see
more bubbles of hydrogen being produced.
This is because the strong acid has more hydrogen ions present in solution
even though it is at the same concentration as the weak acid.
A piece of magnesium the same size should be used
in both acids for the comparison to be fair.

The weak acid will produce the same amount of hydrogen as the strong acid
from the same amount of magnesium