Find the open interval('s) on which / (x) = - 5x* + 50x # 8 is increasing or decreasing. increasing on ( " - , 10); decreasing on (10, co ) () increasing on ( # co . 10); decreasing on (16. zo) O increasing on (" co , 80); decreasing on (80," zo ) increasing on (# co , 40); decreasing on (40, po) increasing on ( - co ; 5); decreasing on (5. = )