If a towel soaks up water, does it gain mass, if not explain why?

The towel does gain mass, since the water is adding a little more weight to the towel. Therefore, the towel may even feel slightly heavier than it was before.

Hope this helps! :D

d) the number of neutrons plus the number of protons

see the picture for the vector

the magnitude of the vector by pythagorean theorem

r = √( (-10)^2 + (3)^2 ) = √109

magnitude = √109 units

the direction angle between vector and x-axis is

tan^(-1)(3/10) = 16.699

so the direction is approximately 16.7 degrees north of west