Step 1: Use the range formula for projectile motion.
For a projectile launched and landing at the same level, the horizontal range is
\[
R=\frac{u^2\sin 2\theta}{g}
\]
Here,
\[
R=90\,\text{m}
\]
and
\[
g=10\,\text{m s}^{-2}
\]
Step 2: Find condition for minimum velocity.
For a given range \(R\), the velocity \(u\) will be minimum when \(\sin 2\theta\) is maximum.
The maximum value of
\[
\sin 2\theta
\]
is
\[
1
\]
So,
\[
R=\frac{u^2}{g}
\]
Step 3: Substitute the given values.
\[
90=\frac{u^2}{10}
\]
\[
u^2=900
\]
\[
u=30\,\text{m s}^{-1}
\]
Step 4: Final conclusion.
Therefore, the minimum velocity of the projectile is
\[
\boxed{30\,\text{m s}^{-1}}
\]