Assuming these two angles form a supplementary angle, - which equals °180 - you should be able to subtract the known angle from 180: 180 - 125 = 55. The second angle should measure °55 degrees.
Hope this helps,
♥<em>A.W.E.</em><u><em>S.W.A.N.♥</em></u>
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<u><em>(</em></u><em>P.S. Alt + 0176 = °</em><u><em>)</em></u>