How to estimate:
Well we know that 432,000 and 239,500 produce a sum of 671500, so to find the sum of 432, 068 and 239, 576, simply add 68 and 76 to 671500:
671500 + 68 + 76= 671644
Therefore, 432,068 and 239,576 make a sum of 671644.
Hope this helps!