Question 201354
25 * 17 
.
12,,,,34,,,,,even(12),  not  add
6,,,,,68,,,,,even (6),  not  add
3,,,,,136
1,,,,,272
.
,,,,,,408  (136 + 272) ,,,,,close  to  425
.
.
48  *  39
.
24,,,,,78,  even(24), not  add
12,,,,,156, even(12),  not  add
6,,,,,,312,  even(6),  not add
3,,,,,,624,
1,,,,,,1248
.
,,,,,,,1872,,(624 +1248),,,,exactly  1872
.
.
120  *  42
.
60,,,,,,84, even (60),  not  add
30,,,,,168,  even(30), not  add
15.,,,,336
7,,,,,,672
3,,,,,1344
1,,,,,2688
.
,,,,,,5040,,,,(336+672+1344+2688),,,exactly 5040
.
again,  first  column divide  by  2,(drop remainders),,second  column multiply  by  two.  Cross  out  even  numbers  in  first  column , and  mates  in  second  column.  Then  add  remaining  second  column.