How is the area of a trapezium calculated?

Here is one way.

Find the average of the two parrallel sides (i.e. the top and bottom), and then multiply that by the height. That's it.

To find the average of the parrallel sides, add them to gether and divide by two.

Example: average of 4 and 6 = (4+6) divided by 2. Which equals 10 divided by 2 which is 5. That's it. And 5 looks right to because its in between 4 and 6.