Let r be the monthly rate of change of Element X, expressed as a decimal. We can then write the function to represent the mass of the sample after t years as:
m(t) = 490(1 + r)^(12t)
This function takes into account that there are 12 months in a year, so we need to raise the quantity (1 + r) to the power of 12t to account for the number of months that have passed.
To find the percentage rate of change per month, we can use the following formula:
percentage rate of change per month = r * 100
For example, if r = 0.02, then the function to represent the mass of the sample after t years would be:
m(t) = 490(1 + 0.02)^(12t)
And the percentage rate of change per month would be:
percentage rate of change per month = 0.02 * 100 = 2%
Note that the rate of change could be positive or negative depending on the decay or growth of Element X, but the formula above assumes a positive growth rate.