The question explicitly asks how many of R were used.

If 6 of R were used, in 1hr, 6 machines would have done 6 x [1/36] = 1/6 of of the work
In 2 hours they would have done 2 x 1/6 = 1/3 of the work

Similarly, if 6 of S were used in 1 hr they would have done 6 x [1/8] = 1/3
In 2 hours = 2/3 of the work

Total work done / 1/3 + 2/3 = 1 complete work

Hence 6 of R and 6 of S were used.