Find four numbers to put in the corners of the square.
Do it so that the total along each line of the square is the same as the total of the numbers in the corners.
Take any trapezium.
Make one straight cut across it and reassemble the pieces to make a triangle.
How whould the cut be made?
In how many different ways can you do this so that each triangle is a different shape?
Since the total of A +B+C+D is the same as the total of B+C+ 7, it follows that A+D= 7, and each line must total 20. Working systematically, try values for A and D such that A +D= 7. The values A = 3, B= 6, C = 7 and D = 4 produce the required result.
Two different ways: join B to the mid-point of AD, or join A to the mid-point of BC