13. For a certain project, if contracted to Team A and Team B, it will be completed in $2 \frac{2}{5}$ days, costing 180,000 yuan; if contracted to Team B and Team C, it will be completed in $3 \frac{3}{4}$ days, costing 150,000 yuan; if contracted to Team A and Team C, it will be completed in $2 \frac{6}{7}$ days, costing 160,000 yuan. Now the project is to be contracted to a single team, under the condition of ensuring completion within a week, which team will have the lowest cost?
Three, 13. Let the number of days required for A, B, and C to complete the task individually be \( x, y, z \) respectively. Then,
$$
\left\{\begin{array} { l } 
{ \frac { 1 } { x } + \frac { 1 } { y } = \frac { 5 } { 1 2 } . } \\
{ \frac { 1 } { y } + \frac { 1 } { z } = \frac { 4 } { 1 5 } , } \\
{ \frac { 1 } { z } + \frac { 1 } { x } = \frac { 7 } { 2 0 } . }
\end{array} \text { Solving, we get } \left\{\begin{array}{l}
x=4, \\
y=6, \\
z=10 .
\end{array}\right.\right.
$$

Let the daily cost for A, B, and C working individually be \( u, r, \mathbb{u} \) yuan respectively. Then,
$$
\left\{\begin{array} { l } 
{ \frac { 1 2 } { 5 } ( u + v ) = 1 8 0 0 0 0 , } \\
{ \frac { 1 5 } { 4 } ( r + u ) = 1 5 0 0 0 0 , } \\
{ \frac { 2 0 } { 7 } ( u + u ) = 1 6 0 0 0 0 . }
\end{array} \text { Solving, we get } \left\{\begin{array}{l}
u=45500, \\
r=29500 \\
u=10500 .
\end{array}\right.\right.
$$

Thus, the cost for A to complete the task alone is
$$
45500 \times 4=182000 \text { (yuan); }
$$

The cost for B to complete the task alone is
$$
29500 \times 6=177000 \text { (yuan); }
$$

And C cannot complete the task within a week.
Therefore, the cost for B to complete the task is the lowest: