“There were 50% more men than women… at first.”

This means that the ratio of men to women at first was 3 : 2.

“Now, there are 20% more women than men [in the end].”

This means the ratio of men to women in the end is 5 : 6.

Multiple both columns by 6 and 5 respectively to get equal parts:

18 units – 600 = 10 units +400
18 units – 10 units = 400 + 600
8 units = 1000
1 unit = 125
3 units = 375 men at first