A male black widow spider is half the size of a female black widow spider. Together, they are 21 millimeters in lenght. How big is a female black widow? A male?

Let x be the size of the male and y the size of the female.

x+y=21 and x=y/2
solve simple simultaneous

To find the size of the female black widow spider, let's assume the size of the male black widow spider is "x" millimeters. According to the given information, the male spider is half the size of the female spider.

So, the size of the female spider can be represented as 2x millimeters.

Now, since the sum of their lengths is 21 millimeters, we can create an equation:

x (size of the male spider) + 2x (size of the female spider) = 21

Combining like terms, we have:

3x = 21

To solve for x, we divide both sides of the equation by 3:

x = 21 / 3

x = 7

Therefore, the size of the male black widow spider is 7 millimeters.

Now, to find the size of the female black widow spider, we substitute the value of x back into the equation:

2x = 2 * 7

2x = 14

Hence, the size of the female black widow spider is 14 millimeters.