$8.40 is the amount to be subtracted from the $42.00. If you multiply 20% (.20) by $42.00 you get $8.40. To get your final price subtract $8.40 from $42.00 to get you final answer. I hope that helps.
Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles.[2]:1.1 It is the foundation of all quantum physics including quantum chemistry, quantum field theory, quantum technology, and quantum information science.
