It would help if you would post the complete question not just a few points in a table. Without all the information provided in the question ( initial value??); I can not be certain of the solution. Looking at the table, you can tell the rate is not constant, it changes between points. The data best fits an exponential decay graph. When plotted in excel it gives the equation:

with a perfect R² = 1
This equation shows that whatever this data represents started at 150 and exponentially decayed/decreased at a rate of -0.223