Th7269, you are bang on.

Iqkhanuk36, Here the only variable is rural mortality rate; all the others are already given.

The formula to work the rural mortality rate would be:

{(urban pop. ratio X urban mortality rate) + ( rural pop. ratio x rural mortality rate)} / (urban pop. ratio + rural pop. ratio) = average mortality rate (please note X = multiplication and / = division)

Now replace the given ones:

=> {(1 x 9.7) + (4.5 X rural mortality rate)} / (1+4.5) = 8.8

=> 9.7 + 4.5 X rural mortality rate = 8.8 X 5.5 (8.8 X 5.5 = 48.4)

=> 4.5 X rural mortality rate = 48.4 - 9.7

=> rural mortality rate = 38.7 / 4.5

therefore, rural mortality rate = 8.6

Hope, it is clearer.