Answer:
[tex]\large\boxed{u=4}[/tex]
Step-by-step explanation:
[tex]s=\dfrac{1}{2}(u+v)t\\\\\text{Substitute the values of}\ s,\ v\ \text{and}\ t\ \text{to the equation:}\\\\s=360,\ v=20,\ t=30\\\\\dfrac{1}{2}(u+20)(30)=360\qquad\text{multiply both sides by 2}\\\\2\!\!\!\!\diagup^2\cdot\dfrac{1}{2\!\!\!\!\diagup_1}(u+20)(30)=2\cdot360\qquad\text{use distributive property}\\\\30u+600=720\qquad\text{subtract 600 from both sides}\\\\30u=120\qquad\text{divide both sides by 30}\\\\u=4[/tex]
