a) The concentration, x grams per litre, of sugar in a solution at time , t minutes is given by the differential equation: dx/dt = 20-3x/30. if initial concentration was 2 grams per litre (that is, when t=0, x = 2), find the time taken for the sugar concentration to rise to 6 grams per litre.

b) The demand for a good decay is, according to the differential equation: dQ/dt = -KQ

where K is a constant and Q is the quantity at time t seconds. Given Q = 50 when t = 0 and that Q = 25 when t = 25 find the time t at which Q = 10.