How many apples in the chef's trays

A chef was arranging trays of fruit on the tables.
Four trays contained a total of 84 apples.
The fifth tray contained 4 fewer apples than the average of the five trays.

QUESTION: How many apples were in the fifth tray?

Could you please explain and show the steps to solve this problem. Much appreciated.

Thanks
.

Call the numbers in the five trays A,B,C,D,E.

Convert the sentences to equation form:

A+B+C+D = 84
E + 4 = (A+B+C+D+E)/5

Substituting:

E + 4 = (84 + E)/5
5E + 20 = 84 + E
4E = 64
E = 16

To confirm: total number of apples is 84 + 16 =100
Average of all five trays = 100/5 = 20
Tray E has four fewer than that average: QED

4A=84
A = 21
B = 21 - 4 = 17 <--