The grid-in failure mode that arises because the answer box has no space bar: typing 1 1/2 is read by the machine as 11/2 = 5.5, so a correct answer of 1.5 is recorded as flatly wrong with no partial credit. Convert every mixed number before it reaches the box — 1tfrac12 to 3/2 or 1.5. The book calls it the single most common way to lose a grid-in you had already earned.