The half-life is the time it takes for any amount of Cobalt-60 to decay to half the starting amount.
20 years is 4 times the half-life of 5 years. So there would be
(300 g) / 2⁴ = 18.75 g
remaining after 20 years.
In other words, after 5 years, 300 g decays to 150 g.
After another 5 years (total 10 years), 150 g decays to 75 g.
After another 5 years (total 15 years), 75 g decays to 37.5 g.
After another 5 years (total 20 years), 37.5 g decays to 18.75 g.