Anna is computing 266 - 39. To do so, she first says that 266 is close to 270 and 39 is
close to 40, so she starts out computing 270 - 40 = 230. She then computes 230 - 3 =
227, and declares that 227 is her final answer. Is Anna's strategy correct? If yes, explain
why. If no, explain why not. Be mathematically specific, and support your reasoning
with a number-line model.