Answer :
[tex]n[/tex] - the number of green blocks which is equal to the number of favourable events
[tex]1+3+n=4n[/tex] - the number of all blocks, which is equal to the number of all possible outcomes
[tex]P(A)=\dfrac{n}{4+n}=\dfrac{1}{2}\\ 1\cdot( 4+n)=2\cdot n\\ 4+n=2n\\ 2n-n=4\\ n=4 [/tex]
[tex]1+3+n=4n[/tex] - the number of all blocks, which is equal to the number of all possible outcomes
[tex]P(A)=\dfrac{n}{4+n}=\dfrac{1}{2}\\ 1\cdot( 4+n)=2\cdot n\\ 4+n=2n\\ 2n-n=4\\ n=4 [/tex]