The extra <br> is coming from the Wordpress text editor you are using. It assumes (rightly) that a line break in your text should be replaced with a <br> in the final markup. (You mentioned you were using $$ signs, but actually, the displayStyle is coming from the fact you are using \tags in those equations.)
One way out of this is to not have any line break after each line involving \tag, like this.
$\therefore \dfrac{{30 \times m \times 18 \times 7}}{{45 \times w \times d \times 9}} = \dfrac{{12t}}{{32c}} \tag{1}$ It is given, 4 men can make 3 tables in the same time as 3 women can make 4 chairs.
That is not so satisfactory. It may be too late for your situation, but I feel it's always better to use <p> tags rather than <br> for line breaks, and also to use margin rather than padding between lines. Then (for example) the (default) 1em margin at the bottom of the first <p> (the one with an equation) would "merge" with the (default) 1em margin at the top of the second paragraph (the one with text), resulting in a (nicer) 1 em spacing between lines.
Then you would have spacing that is easier to read (where lines within paragraphs have a single line space, and between paragraphs have a double line space), like this:
We cannot directly apply chain rule here as work is different in both cases as well as on the first job men are working and on the second women are working. ...Filler text. Filler text. Filler text. Filler text. Filler text. Filler text....
Let the efficiency of a man = m, Woman = w
Most WYSIWYG editors produce a <p> tag if you do 2 <enter>s at the end of a line (and a <br> for only one). Try it with 2 and check it gives <p>s, and see how it looks. You may need to change your CSS to suit.
Regards
Murray