Why were mechanical calculators developed?

The mode is generally used when delivering a presentation to an need now

Need asap write a short paper describing the history and differences between six sigma, waterfall, agile, and scrum models. understanding these models can give you a good idea of how diverse and interesting it development projects can be. describe what the rationale for them is and describe their key features. describe the history behind their development. at least 400 words

Awide variety of “ apps “ are available to customize devices. which category of app does the word processing software fall into?