The model dy/dt = ky describes exponential growth (k > 0) or decay (k < 0). Solution: y = $y_0$ * e^(kt) where $y_0$ is the initial value. Half-life: t_$\frac{1}{2}$ = ln(2)/|k|. Doubling time: $t_{double}$ = ln$\frac{2}{k}$. Newton's Law of Cooling: dT/dt = -k(T - $T_s$) where $T_s$ is the surrounding temperature. This is a linear DE: dT/dt + kT = $kT_s$. IF = e^(kt). Solution: T = $T_s$ + ($T_0$ - $T_s$)*e^(-kt). In JEE, these are typically word problems where you set up the DE, solve it, and use given conditions to find constants.