The mixture of A1,A2,A3 type seeds is 3:5:2.
We have, P(A1)=310=0.3P(A2)=510=0.5P(A3)=210=0.2
Let E be the event that a seed germinates, then
P(E | A1)=0.4P(E | A2)=0.6P(E | A3)=0.4
By using 'Theorem of total probability', we have
P(E)=P(A1)×P(E | A1)+P(A2)×P(E | A2)+P(A3)×P(E | A3)
=0.3×0.4+0.5×0.6+0.2×0.4
=0.5