Question 1199236
B, the base bill amount before tax, before tip.


NEED to know the tax rate.   Why you not include this value?  Assuming 8% tax rate,
{{{B+0.08B=76.80}}}
-
{{{(1.08)B=76.80}}}

{{{B=76.80/1.08}}}


The 15% tip should be close-to  {{{0.15(76.8/1.08)}}} dollars.