Application of Differential Equations

The curve in which the slope of the tangent at any point equals the ratio of the abscissa to the ordinate of the point is

The slope of a curve at any point is the reciprocal of twice the ordinate at that point and it passes through the point (4, 3). The equation of the curve will be

The population of a country doubles in 40 years. Assuming that the rate of increase is proportional to the number of inhabitants, the number of years in which it would treble itself is: