In humans, widow's peak (w) is dominant over straight hairline (w). if a heterozygous male marries a female with a straight hairline, what percent of their children can be expected to have widow's peak?

Widow's peak is heterozygous Widow's peak, so the genotype would be Ww. The female has straight hair which is recessive, so she would have the genotype ww.

If we put that in a Punnet square, it will look like this:

      W      w
w  Ww    ww
w  Ww    ww

Because Widow's peak is dominant it will be expressed. So if you have a gentoype with Ww, Widow's peak will still be expressed. So since there are two genotypes in the square out of the four, then the percentage is 50%.