If you take a look at what are all these numbers here that we see, you will notice that they are all cube numbers. That is, each of them is some kind of number, multiplied by itself and then by itself again. 8 is 2 times 2 times 2. 27 is 3 times 3 times 3. 125 is a cube of 5 and 216 is a cube of six. 1 is a cube (or any other degree of 1) of 1. So we have here, a cubic one, a cube of 2, then three, then a gap, then five… which logically means we’re missing a cube of 4 in this sequence. And 4 times 4 times 4 is 64. This is our missing number.