The chemical element einsteinium-253 naturally loses its mass over time. A sample of einsteinium-253 had an initial mass of 320 grams when we measured it.
The relationship between the elapsed time t, in days, and the mass, M(t), in grams, left in the sample is
modeled by the following function:

M(t) = 320 - (0.125)^t/61.4
Complete the following sentence about the rate of change in the mass of the sample.

The sample 87.5% of its mass every days