15. Which of the following lines are parallel to each other? Explain.
\[
y=-3 x+10
\]
\[
y=\frac{2}{5} x-8
\]
\[
-5 x+2 y=-16
\]
\[
2 x-5 y=30
\]
\[
6 x+2 y=-100
\]

Step 1 ：Given the equations of the lines, we need to find which ones are parallel. Two lines are parallel if and only if their slopes are equal.

Step 2 ：The slope of a line in the form \(y = mx + b\) is \(m\). For lines not in this form, we can rearrange them to this form to find the slope.

Step 3 ：Rearranging the equations to the form \(y = mx + b\), we get: \[y = -3x + 10\], \[y = 0.4x - 8\], \[y = 2.5x - 8\], \[y = 0.4x - 6\], \[y = -3x - 50\]

Step 4 ：The slopes of the lines are -3, 0.4, 2.5, 0.4, and -3 respectively.

Step 5 ：Therefore, the lines with equations \(y = -3x + 10\) and \(y = -3x - 50\) are parallel to each other because they have the same slope.

Step 6 ：\(\boxed{\text{The lines } y=-3 x+10 \text{ and } 6 x+2 y=-100 \text{ are parallel to each other.}}\)