2X-8=0.5(X+8)+9

2X=0.5X+4+ 17

4X=X+42

3X=42

X= 14

The number of people in group B is 14.

It is estimated that it will take x hours to complete.

1/ 15+4 * 1/ 12+X *( 1/ 15+ 1/ 12)= 1

1/ 15+ 1/3+X[(4+5)/60]= 1

6/ 15+X[3/20]= 1

2/5+X[3/20]= 1

X[3/20]=3/5

X=(3/5)/(3/20)

X=(3/5)*(20/3)

X=4

It will take another four hours to finish.