Question 172325
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{{{drawing(350,350,-2,1,-1,2, triangle(-1.191753593,0,.1763269807,0,0,1),
locate(-1.2,0,A), locate(.2,0,B), locate(0,1.14,C)  

)}}}

Let 

{{{A}}} = first angle,
{{{B}}} = second angle
{{{C}}} = third angle
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the measure of the first angle is one-half the measure of the second angle.
<pre><font size = 4 color = "indigo"><b>
So {{{A = 1/2}}}{{{B}}}
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The third angle's measure is 20° less than the measure of the second angle 
<pre><font size = 4 color = "indigo"><b>
So {{{matrix(1,3,C,"=","B-20°")}}}

Since {{{matrix(1,7, A, "+", B, "+", C, "=", "180°")}}}

{{{1/2}}}{{{matrix(1,7,      B,  "+", B, "+", "(B-20°)", "=", "180°"   )}}}  

{{{1/2}}}{{{matrix(1,9,      B,  "+", B, "+", B, "-", "20°", "=", "180°"   )}}}

{{{1/2}}}{{{matrix(1,7,      B,  "+", 2B, "-", "20°", "=", "180°"   )}}}

Add {{{"20°"}}} to both sides:

{{{1/2}}}{{{matrix(1,5,      B,  "+", 2B,  "=", "200°"   )}}}

Multiply every term by 2 to clear of fractions:

{{{matrix(1,2,"2*",1/2)}}}{{{matrix(1,5,      B,  "+", 2*2B,  "=", "2*200°"   )}}}

{{{matrix(1,3,cross(2),"*",1/cross(2))}}}{{{matrix(1,5,      B,  "+", 2*2B,  "=", "2*200°"   )}}}

{{{matrix(1,5,B, "+", 4B, "=", "400°" ) }}}

{{{matrix(1,3, 5B, "=", "400°")}}}

{{{matrix(1,3, B, "=", "80°")}}}

{{{A = 1/2}}}{{{matrix(1,6,     B, "=", 1/2, "*80°", "=", "40°") }}}

{{{matrix(1,7,C,"=","B-20°", "=", "80°-20°", "=","60°"  )}}}

{{{drawing(350,350,-2,1,-1,2, triangle(-1.191753593,0,.1763269807,0,0,1),
locate(-1.2,0,A), locate(.2,0,B), locate(0,1.14,C), locate(-.09,.15,"80°"),
locate(-1,.15,"40°"), locate(-.16,.85,"60°")  )}}}

Edwin</pre>