Question 21

Suppose that there are $100$ people in the army.

In total there are $70+75+80+85=310$ injuries.

The least number of people with all four injuries occurs when the injuries are as evenly distributed amongst people in the army as possible.

Now, $300\div100=3.1$, so each person would need to have at least $3$ injuries.

Since this would result in $300$ injuries in total, $10$ people must have $3+1=4$ injuries in this scenario.

$\therefore\enspace$at least $10\%$ must have had all four injuries.