How many factors should a well designed experiment test at one time

There should be at least two factors tested for a good expermient 

Simple multiply 45 units by $0.99you'll get $44.55

If you decide to make the leap to horse ownership it truly is a life changing event. once you sign the dotted line on the bill of sale you will have full responsibility for care, custody, training and financing your new horse.  . and leasing is just for a period of time for a amount of )