Differential equations — practice question

A field contains 300 plants of one species, and every one of them may catch a particular disease. Let x be the number infected at time t after the first plant becomes infected. The rate at which x changes is proportional to the product of the number already infected and the number not yet infected. The variables x and t are regarded as continuous, and it is given that $\frac{dx}{dt} = 0.2$ and $x = 1$ when $t = 0$.