Chemistry and Chemical Reactivity (9th Edition)

$t=2.45\ h$
For a first-order reaction; $ln(C/C_0)=-k.t; k=0.693/t_{1/2}$ Assuming constant volume, we can use this equation on a mass basis. $ln(0.25\ mg\div 7.50\ mg)=-0.693/30\ min\times t$ $t=147.2\ min=2.45\ h$