$3.)$ The table below shows the electric current in a circuit I $($in milliamperes$)$ as a function of time $t($in seconds after a switch in the circuit is closed$).$

$\backslash$table$[[t(s),0,5,10,20,25,40],[I(mA),41,55,70,84,93,102]]$

a$)(4$ pts$.)$ Estimate $I^{\text{'}}(15)$ and $I^{\text{'}}(2\mathrm{.}5).$

b$)(1$ pts$.)$ Using appropriate units, explain the meaning of your estimate for $I^{\text{'}}(2\mathrm{.}5).$