To meet a government requirement, a bottler must test 5 percent of its spring water and 10 percent of its sparkling water for purity. If a customer ordered 120 cases of spring water and 80 cases of sparkling water, n what percent of all cases must bottler test before he can send it out ?

Number of cases required to be tested :
$$\eqalign{ & = 5\% \,\,{\text{of}}\,\,120 + 10\% \,\,{\text{of}}\,\,80 \cr & = \left( {\frac{5}{{100}} \times 120} \right) + \left( {\frac{{10}}{{100}} \times 80} \right) \cr & = 6 + 8 \cr & = 14 \cr} $$
∴ Required percentage :
$$\eqalign{ & = \left( {\frac{{14}}{{120 + 80}} \times 100} \right)\% \cr & = \left( {\frac{{14}}{{200}} \times 100} \right)\% \cr & = 7\% \cr} $$