61% of 3400 people at a concert were men. how many were women?

We will assume that there are only men and women present.
Per cent is a ratio - it describes the amount "per hundred".
So if 61% are men, the remainder must be women.
The remainder can be calculated by taking the whole (100%) and taking away the amount we know are men (61%)
100% - 61% = 39%.
To work out the percentage, multiply by the ratio amount and then divide by 100.

3400 * 39 / 100 

We can simplify this straight away because the 3400 can cancel out the divide by 100 to give us 34.

34 * 39 = 1326