Estimating Differences: The Star Tower

At the Star Tower, Professor Luna Moon keeps jars of gold stars and silver stars and counts the stars she spots every night. To know about how many more, she rounds both numbers and takes away.

Remember: Estimate a difference: round each number, then subtract. 812 − 394 is about 800 − 400 = 400. 73 − 28 is about 70 − 30 = 40. Round both numbers, and round them to the same place.

Magic trick: Round first, then take away the round numbers: 800 − 400 is as easy as 8 − 4 with two zeros on the end.