The radio activity of a sample is $'X'$ at a time $'t_1'$ and $'Y'$ at a time '$t_2'$. If the mean life time of the specimen is 't', the number of atoms that have disintegrated in the time interval $(t_1-t_2)$ is :

Question

$$\begin {array} {1 1} (1)\;Xt_1-Xt_2 & \quad (2)\;X-Y \\ (3)\;\frac{X-Y}{t} & \quad (4)\;(X-Y)t \end {array}$$