True/False Question:
Let $u_{1},u_{2},u_{3},u_{4}$ be vectors in $\mathbb{R}^{2}$ and
$$u=\sum_{j=1}^{4}t_{i}u_{j}\:\:\: ; \:\:\:t_{j}> 0 \:\: and \sum_{j=1}^{4}t_{j}=1.$$
Then three vectors $v_{1},v_{2},v_{3}\in \mathbb{R}^{2}$may be chosen from $\left \{ u_{1},u_{2},u_{3},u_{4} \right \}$
such that
$$u=\sum_{j=1}^{3}s_{i}v_{j}, \:\:\:s_{j}\geq 0 ,\:\: \sum_{j=1}^{3}s_{j}=1.$$
