6*4*5-100=20 50*9*2-80=820 2*7*25+150=500 15*9*4-540=0

Great job! Let's go over the solutions to each equation to make sure everything is correct:

1. 6 * 4 * 5 - 100 = 20 The multiplication is done first: 6 * 4 = 24, then 24 * 5 = 120. Finally, subtract 100 from 120: 120 - 100 = 20.

2. 50 * 9 * 2 - 80 = 820 Multiply 50 * 9 = 450, then 450 * 2 = 900. Finally, subtract 80 from 900: 900 - 80 = 820.

3. 2 * 7 * 25 + 150 = 500 Multiply 2 * 7 = 14, then 14 * 25 = 350. Finally, add 150 to 350: 350 + 150 = 500.

4. 15 * 9 * 4 - 540 = 0 Multiply 15 * 9 = 135, then 135 * 4 = 540. Finally, subtract 540 from 540: 540 - 540 = 0.

Well done! All the solutions are correct. Keep up the great work in your exam preparation! If you have any more questions or need further assistance, feel free to ask.