10. A store has 5 bags of flour, each weighing between 25 to $30 \mathrm{~kg}$. The store has a scale, but it only has weights that can measure $50 \sim$ $70 \mathrm{~kg}$. Now, to determine the weight of each bag of flour, at least ( ) times of weighing are needed.
(A) 4
(B) 5
(C) 6
(D) 7
10. B. $x \mathrm{~kg}, y \mathrm{~kg}, z \mathrm{~kg}$, two items are weighed together, the recorded weights are $a \mathrm{~kg}, b \mathrm{~kg}, c \mathrm{~kg}$, then the system of equations is
$$
\left\{\begin{array}{l}
x+y=a, \\
y+z=b, \\
z+x=c
\end{array}\right.
$$

To find $x, y, z$, you can weigh two items together with one of the known items, and then solve for their weights. Therefore, you need to weigh 5 times.