His measurements could be as much as 0.1 cm wrong.
He measures 3 cm by 4 cm .
-- The smallest the area could possibly be is if both of his measurements
were too big. The rectangle would actually be 2.9cm by 3.9cm, and the
area would actually be (2.9 x 3.9) = 11.31 square-cm.
-- The biggest the area could possibly be is if both of his measurements
were too small. The rectangle would actually be 3.1 cm by 4.1 cm, and
the area would actually be (3.1 x 4.1) = 12.71 square-cm.
So the true area must be between 11.31 ≤ Area ≤ 12.71 .