The easiest way would be to say 117 billion is ~2^37 because 2[1] are 128, 1024, and about a million (1024*1024), so multiplying those all together gives a little over 128 billion, which is pretty close to 117 billion. So, 242/237=2^5=32. Pretty close, all mental math. Granted, it does require you to either memorize or compute powers of 2 up to 10.
getting correct within a factor of 2 after a 42-fold exponentiation would be amazingly good mental math
The easiest way would be to say 117 billion is ~2^37 because 2[1] are 128, 1024, and about a million (1024*1024), so multiplying those all together gives a little over 128 billion, which is pretty close to 117 billion. So, 242/237=2^5=32. Pretty close, all mental math. Granted, it does require you to either memorize or compute powers of 2 up to 10.
7, 10, 20 ↩︎