I must be going mad but I can't remember how to work out the cost for a box quinella.

I thought if there were 5 runners, the price would be 5*4 = 20 for $1.
6 runners, 6*5 = 30
7 runners, 7*6 = 42

Now if that were the case with a makeup of :
4 races with 5 runners = 4 * 20 = 80
4 races with 6 runners = 4 * 30 = 120
2 races with 7 runners = 2 * 42 = 84

Total outlay of 284 or 142 for 50 c.....