Question 153903
Based on how you expressed your formula -- it could be interpreted in one of two ways -- I'll show you BOTH cases.  (I think you mean Case #1)
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Case #1, if you mean:
{{{ (1/5)b = -8 }}}
multiply both sides by 5:
{{{ b = -40 }}}   (answer for case 1)
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Case #2, if you mean:
{{{ 1/(5b) = -8 }}}
multiply both sides by 5b:
{{{ 1 = -40b }}}
divide both sides by -40:
{{{ -1/40 = b }}}   (answer for case 2)