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it helps to know which pairs of envelopes were paired together to get the weights.
i will assume the following:
a + b = 59
a + c = 61
a + d = 62
b + c = 63
b + d = 64
c + d = 66
based on this analysis:
envelope d would be the heaviest at 33.5 grams.
envelope c would be the next heaviest at 32.5 grams.
envelope b would be the next heaviest at 30.5 grams.
envelope a would be the lightest at 28.5 grams.
the method of solution is to determine that:
3a + 3b + 3c + 3d = 375 grams.
you also know that:
b = 59 - a
c = 61 - a
d = 62 - a
substitute in the totals equation to solve for a.
once you solve for a, the rest follow.
your total equation becomes:
3a + 3(59-a) + 3(61-a) + 3(62-a) = 375
simplify to get:
3a + 177 - 3a + 183 - 3a + 186 - 3a = 375
combine like terms to get:
-6a + 546 = 375
subtract 545 from both sides to get:
-6a = 375 - 546 = -171
divide both sides by -6 to get:
a = 28.5
this leads to:
b = 59 - 28.5 = 30.5
c = 61 - 28.5 = 32.5
d = 62 - 28.5 = 33.5
the heaviest envelope is envelope d at 33.5 grams.