Nie jest źle, ale można co nieco dopracować
>We used to sell 250 mobiles per month until May PRZECINEK when sales increased
>dramatically by almost 20% PRZECINEK GOING [bo co to znaczy 'and': wzrost o 20% był niezależny od wzrostu z 210 na 270?] up from 210 to 270.However,there
>was a slight decrease in July.The amount of mobiles
>sold rose sharply to 300 in August, DECREASING GRADUALLY the next month TO remain stable at 280 SUBSEQUENTLY.