A word ladder (also known as doublets) is a word game invented by Lewis Carroll. A word ladder puzzle begins with two words, and to solve the puzzle one
must find a chain of other words to link the two, in which two adjacent words differ by one letter. Here is an example:
Change TEN to SIX.
T E N
T I N
S I N
S I X
Here are four math doublets proposed by Roger Robison. You must solve three of them to get credit for this month's problem.
Change ZERO to NULL
Change ONE to TWO
Change FOUR to FIVE
Change TAN to ARC