Mathematics

Problem

For

0\le t\le 4\,\text{min}

, a quantity

Q

satisfies

\frac{dQ}{dt}=0.15Q

. At

t=4\,\text{min}

, the conditions change and for

t>4\,\text{min}

the quantity follows

\frac{dQ}{dt}=-0.05Q

. Determine the factor

\dfrac{Q(7)}{Q(0)}