On the table, there is a tower made of blocks numbered from 1 to 90. Bob takes blocks from the top of the tower, 3 at a time, to build a new tower as shown. When he has finished building the new tower, how many blocks will be between the blocks numbered 39 and 40? So, when we read from the bottom of the new block, we see that we have 88, 89, 90 and then we almost restart, going to 85, 86, 87 and then we restart again, 82, 83, 84, continuing all the way up until we get to 1, 2, 3. Note that every time we have a multiple of 3 in the new stack, if we have 3x, for example 90, then below it we have 3x-1, 89, and 3x-2, 88. Then, because we seek the blocks between 39 and 40, we know that beneath 39, there will be blocks 38 and then 37. Then, following the pattern from earlier, where there was a jump every time and it decreased by 5, then under block 37, we know there will be block 42. Under that, because it's a multiple of 3, we know it will be 41 and then 40. Then, reading from bottom to top, we have block 40, 41, 42, and then 37, 38, and then 39. Therefore, blocks 41, 42, 37, and 38 are between blocks 39 and 40, and our answer is E, 4 blocks.

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