Layout is a set of marks you can plan. Yours faithfully follows Dear Sir. Yours sincerely follows a name.
The idea
A formal letter carries your address, the date, the recipient's designation and address, a salutation, a body in paragraphs, a complimentary close and your name. If you do not name the person, salute "Dear Sir" or "Dear Madam" and close "Yours faithfully". If you name them, salute "Dear Dr Mensah" and close "Yours sincerely". The first body paragraph states why you are writing. Later paragraphs give the facts in the order a busy reader needs. The last paragraph says what you want done, once, politely.
An informal letter keeps your address and the date, drops the formal recipient block, and uses a personal salutation and close. Do not decorate a formal letter with "How is the family?". That sentence belongs to a friend. The body is still English: paragraphs, a point each, no slang. On the real paper the letter is long enough to show development. The questions here test the decisions that lose layout marks before a marker even reads the argument.
Short annotated examples of a formal letter, an informal letter, an article, a speech, a report and a story are on the formats guide. WASSCE essay and letter formats