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A)  Front-end technique (rounding off the leftmost digit of the addends)
     368  ≈  400 (3 is the leftmost digit, rounded off to 4 since the digit to its left is
                                                             more than 4))
+  231 ≈   200  (2 is the leftmost digit)
                 600 Sum

b)  Estimate by rounding off  to the largest place value of each addend
      368  ≈  400  (hundreds is largest place value)
   + 231 ≈  200   (hundreds is the largest place value)
                   600  sum

2 5 2