



 
Hi Monica, I want to consider $15 + 9 + 15.$ I look for two of the three numbers that I can easily add. I know that $15 + 15 = 30$ so in my mind I think of $15 + 9 + 15$ as $15 + 15$ and then add $9.$ Thus $15 + 9 + 15$ is $15 + 15 = 30$ plus $9$ more which is $39.$ The idea is to do this mentally without the need to write it down. Let's try another, $250 + 35 + 150.$ Here I see that $250 + 150 = 400$ so $250 + 35 + 150 = 400 + 35 = 435.$ I hope this helps,  


Math Central is supported by the University of Regina and The Pacific Institute for the Mathematical Sciences. 