The first part basically asks us to look for the maximum length of the horizontal lines as the horizontal line gives us the population. More the length, more the population. We can see that the two lines between 30 and 40 are the longest. Hence, answer is 30s.
Coming to the second part. Let us understand the graph.
The horizontal line gives the number of males on the left side in blue colour and the number of females on right side in yellow colour. Wherever the number of the two is equal, we have equal length of lines on both side. But incase the number for any one is more, the length becomes bigger there. To give you how much bigger it is in relation to other, a dotted line is put giving the length of the other colour. For example, if you look at 85-90, the yellow portion has dotted line, the length of blue if put on yellow colour would be equal to length till dotted lines.
Thus two cases:-
If No dotted lines- both the lengths are equal.
If a dotted line is there- just check the colour that has the dotted line. The length till dotted lines gives the length of other colour line, while the entire coloured line gives the length of that particular colour. So, clearly the portion of line on the right side of dotted line in yellow colour is excess of yellow over blue, and on the left of the dotted lines in blue colour gives the opposite data.
We are to find the max ratio of females, so look at only yellow colour lines with dots. The excess portion is towards right, so wherever the right of dotted portion is maximum as a % of the left of the dotted line within that colour will be the answer. Here it is 85-90.