Let the 9 members be A, B, C, D, E, F, G, H, I. Now the number of member's ages divided by 9 is equal to 70. So the sum of the ages will be 70*9=630. Now, average age of first 4 members is equal to 64. Therefore the sum of the age of the first 4 members is 64*4=256. Again, average age of the last 7 members is 80. So the sum of the age of the members is 80*7=560. Now put the age value of the first 4 members in the sum of all the 9 members , ie, A+B+C+D+E+F+G+H+I=630.
Now, Put the above found age value in the sum of the last 7 members, ie,
Average of C+D=186/2=93.
So the average age of the third and fourth member is 93.