Over the last 50 years, the average temperature has increased by 2.5 degrees worldwide (i made this up). what is the rate of change in worldwide temperatures per year?

50 years ... 2.5 degrees
1 year ... x degrees = ?

If you would like to know what is the rate of change in worldwide temperatures per year, you can calculate this using the following steps:

50 * x = 2.5 * 1
50 * x = 2.5      /50
x = 2.5 / 50
x = 0.05 degrees per year

Result: The rate of change is 0.05 degrees per year.