400
Bicycles have evolved over many years. In 1818, a heavy two-wheeled device called a "velocipede" was patented in Paris. The rider controlled the direction with a steering wheel and moved it forward by pushing his feet on the ground. It caught on briefly, and many people worked to refine its design. In 1863, pedals were added. A few years later, a new, lighter bike was developed that used a very large front wheel and a small rear wheel. Called the "Ariel," it became popular but was quite accident-prone. Over the next decade, "safety" bicycles were produced, incorporating welcome improvements such as air-filled rubber tires, brakes, and gears. By 1899, about a million bicycles were in use.
a. Sequential
b. Problem and solution
Sequential