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_(1/2) = ln(2)/|k|. Doubling time: t_double = ln(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.