First, recognise that this a "Total unchanged" problem.
(ie. The "giving" is mutual. Hence the total before and after the giving should remain the same!!)
C D
1u : 8u --> notice total is 9u
+35 -35
--------------
2u : 1u --> notice total is only 3, which is different from beginning
Transform into 9u by multiplying by multiplying by 3:
6u : 3u
Comparing before and after the giving,
5u --> 35
1u --> 35/5 = 7
Crystal (at the end) --> 7 x 6 = 42
