Question 1078360
.
Angle B = 180 - 100 = 80.


Learn on Supplementary angles from the lesson

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=http://www.algebra.com/algebra/homework/Angles/Angles-basics.lesson>Angles basics</A> 

in this site.


On solved problems similar to your see the lesson

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=http://www.algebra.com/algebra/homework/Points-lines-and-rays/Solved-problems-on-supplementary-and-complementary-angles-and-pairs-of-angles.lesson>Solved problems on supplementary and complementary angles</A>

in this site.


Also, &nbsp;you have this free of charge online textbook on Geometry

&nbsp;&nbsp;&nbsp;&nbsp;<A HREF=https://www.algebra.com/algebra/homework/Triangles/GEOMETRY-your-online-textbook.lesson>GEOMETRY - YOUR ONLINE TEXTBOOK</A> 

in this site.


The referred lessons are the part if this textbook under the topics 
"<U>Basics of angles. Supplementary, complementary angles. Vertical angles. Parallel lines</U>"   and

"<U>Finding angles of triangles, parallelograms and rectangles</A>".