Abusiness letter is not written a. between a company and its customers or clients b. between one company and another company c. between two private individuals d. to employees within a company to recognize important achievements

C. between two private individuals

Explanation:

The business letter should always be in a written form. In the business letter, the information is about business activities which could be important for business dealing or any other business purpose.  

The information which is mentioned in the business letter is somewhat crucial or we can say business matters and no one wants to disclose it as per the requirements.  

It could be either written from a company to its clients, from one company to another company or between the employees who are working in the same company.

But for two private individuals, the business letter could not be written as there is no specific purpose.

answer; according to the ( irs );

u = 30

continuing your method

= +

subtract from both sides

= - thus

= -

= ⇒ u = 30