You wrote it the proper way according to most authorities.
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It is: 24*12 = 288 inches
You write 8.8 million as 8,800,000
To write 20.5 million numerically, you would write it as 20,500,000.
33.9417 million.
You can write 40.1 million as 40,100,000 or simply 40.1M.