$\mathrm{For}\mathrm{every}3\mathrm{tablets},\mathrm{she}\mathrm{makes}=(180\times 3)+75=\$615\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}\mathrm{To}\mathrm{make}\mathrm{at}\mathrm{least}\$4000\phantom{\rule{0ex}{0ex}}\$4000=(6\times \$615)+(\$310)\phantom{\rule{0ex}{0ex}}\mathrm{So}\mathrm{she}\mathrm{must}\mathrm{sell}\mathrm{at}\mathrm{least}\phantom{\rule{0ex}{0ex}}(6\times 3)+2=20\mathrm{tablets}\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}+2\mathrm{because}2\times \$180=\$360(\$310)$ 


