An archer's arrow hits a straw target at 60 m/s and stops after going 0.30 m into it…

Step 1. Deceleration. $0 = (60\ \text{m/s})^2 - 2a \times 0.30\ \text{m}$, so $a = \dfrac{3600\ \text{m}^2/\text{s}^2}{0.60\ \text{m}} = 6000\ \text{m/s}^2$, that is $6.0 \times 10^3\ \text{m/s}^2$.

Step 2. Time. The average velocity is $30\ \text{m/s}$, so $t = \dfrac{0.30\ \text{m}}{30\ \text{m/s}} = 0.010\ \text{s}$.