Another, and I personaly feel is a stronger way of placing this 6 horse trifecta would be ,

2 sel*4 others*4=24*3=$72
or in other words,
2*4*4
4*2*4
4*4*2
Cost 72 units.@.50 cents=$36
We need one of the 2 anchor sels. to get up (but not both).with 2 of the other 4 getting up in any order.
We have reduced the std. box trifecta cost ($120 ) down to $72 , a massive 40% reduction.

The 2 anchor horses could be the 2 with the best av. prize money .

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Cheers.