Question 84810
solve for b

A=1/2h(B+b)  Area of Trapezoid
This is one way to do it, some people would rather distribute first, but I don't.
{{{2A=2(1/2)h(B+b)}}}
{{{2A=h(B+b)}}}
{{{2A/h=(h/h)(B+b)}}}
{{{2A/h=B+b}}}
{{{2A/h-B=-B+B+b}}}
{{{2A/h-B=b}}}
{{{highlight(b=2A/h-B)}}}

also solve for b
This time I will distribute first.
A=1/2(16)(20+b) 
{{{A=8(20+b)}}}
{{{A=160+8b}}}
{{{A-160=-160+160+8b}}}
{{{A-160=8b}}}
{{{(A-160)/8=8b/8}}}
{{{(A-160)/8=b}}}
{{{highlight(b=(A-160)/8)}}}
Happy Calculating!!!