b) 20 grams of salt are dissolved into a beaker containing 1 litre of a certain chemical. The mass of salt \( M \) grams, which remains undissolved \( t \) seconds later, is modelled by the differential equation \( \frac{d M}{d t}+\frac{2 M}{20-t}+1=0, t \geq 0 \).
Solve the differential equation.