If you have one of Scotts books...look at the formula there
Quinellas....100/(x(y+1)) + 100/(y(x+1) where x and y are the price of the horses...
so if two 4/1 shots.... 100/(4(4+1)) + 100/(4(4+1)) becomes
100/20+ 100/20 equals 10
works the same for exactas....say 6/4 and 10....
100/1.5(10+1) =100/16.5 say 6....
100/10(1.5+1) = 100/25 is 4....
If you are interested I have a chart on the prices which obviates the need for all the mathematics..