Step 1: Identify the gas law to be used.
Since the temperature remains constant, Boyle's law is applicable.
According to Boyle's law,
\[
P_1V_1=P_2V_2
\]
where
\[
P_1=760\ \text{mm Hg},
\]
\[
V_1=100\ \text{cc},
\]
\[
P_2=400\ \text{mm Hg}.
\]
We have to find
\[
V_2.
\]
Step 2: Substitute the given values.
Using Boyle's law,
\[
760 \times 100
=
400 \times V_2
\]
Step 3: Solve for \(V_2\).
\[
V_2
=
\frac{760 \times 100}{400}
\]
\[
V_2
=
190\ \text{cc}
\]
Step 4: Verify the result.
Since pressure decreases from
\[
760\ \text{mm Hg}
\]
to
\[
400\ \text{mm Hg},
\]
the volume should increase.
The calculated value
\[
190\ \text{cc}
\]
is therefore physically correct.
Step 5: Final conclusion.
Hence, the volume of the gas at \(400\ \text{mm Hg}\) pressure is
\[
\boxed{190\ \text{cc}}
\]