3 to 4.....added = 7
3/7x = 210....3/7 of all factory workers (x) are men...and men = 210
x = 210 * 7/3
x = 1470/3 = 490...so there is 490 factory workers
4/7(490) = 1960/7 = 280...and there are 280 women....because 4/7 of all factory workers are women