Question 660854
There are several ways to solve this problem (e.g. as a system of linear equations or by substitution).  However, since we're not looking for each penguin's individual weight, there is a simple way to do it.
Let p by Pesky's weight, w be Waddle's weight, and t be Tux's weight.  Here's what we're given:
{{{p + w = 80}}}
{{{p + t = 56}}}
{{{w + t = 90}}}
Add all three equations together:
{{{p + w + p + t + w + t = 80 + 56 + 90}}}
{{{2p + 2w + 2t = 226}}}
{{{2(p + w + t) = 226}}}
{{{(2(p + w + t))/2 = 226/2}}}
{{{p + w + t = 113}}}
So the combined weight of all three penguins is 113 kg.  Unfortunately, they can't all ride the Crevasse together.