若一个$\alpha$角的终边上有一点$P\left(-4,a\right)$且$\sin \alpha ·\cos \alpha =\frac{\sqrt{3}}{4}$，则a的值为(　　)

小华想帮妈妈做下面的事情：用洗衣机洗衣服要用20分钟；扫地要用6分钟；擦家具要用10分钟；晾衣服要用5分钟．她经过合理安排，做完这些事至少要花（　　）分钟．

   I like my bike. It brings me a lot of fun. When I was an1boy, my father bought it for me. It is my “horse”. I often go to school2 my bike. I like riding a bike 3 it is very convenient. If I go out by bike, it is 4 to find a place for parking it. Bikes are very cheap, so most people can buy one. There are three people in my family. 5of us has a bike. My parents are teachers in my school. In the morning, Mum, Dad and I each 6 to school. However on a rainy day we walk to school, because the road is bad for riding. I think riding bikes can make us7 . It not only helps us exercise more but also 8 the air clean. The bike is very important in Chinese people’s life. So riding bikes can help you to 9 more about China and the Chinese people. Now people in the world say China is the country of bikes because there are 10 bikes in China. I hope people all round the world like them.