How did scientists discover the layers of the earth

The force acting on a beam was measured under the same operating conditions and a sample of 5 data points was collected. this process was repeated by 3 observers. the pooled standard deviations of these 3 data sets was 1.21. determine the true mean force using all pooled data (with a 95% probability). the result should look like this: true mean = mean +/- margin of error

70 give a real life example showing how sensory neurons work with the motor neurons

When using the scientist method, which step comes last?