The question narrows the groups down from 3 groups with men, women and children to 2 groups with adults and children. Combine the ratios of the men and the women to children to create this new ratio: (3+5):2 = 8:2. You can then simplify it to 4:1.
Let x be the number of children.
Cross-multiply the two ratios to get the expression below, simplify and solve for x:
Since there are 120 children, simply add the number of adults (360 more than this value as mentioned before) and you have your final answer: