mml=<math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced open="[" close="]" separators="|"><mrow><mfenced separators="|"><mn>50</mn></mfenced><msup><mn>3</mn><mrow/></msup><mo> </mo><mo>+</mo><mo> </mo><mo>(</mo><mo>-</mo><mn>30</mn><msup><mo>)</mo><mn>3</mn></msup><mo> </mo><mo>+</mo><mo> </mo><mo>(</mo><mo>-</mo><mn>20</mn><msup><mo>)</mo><mn>3</mn></msup></mrow></mfenced></math>