Is an offer valid if the wage is conditional on the majority of shots being hits out of a set number?

Primary text

It is valid if a person says, 'Shoot ten arrows; if your successful shots outnumber your misses, you will have one dirham.' This is valid because the wage is contingent upon a known standard of successful hits; for ten shots, the minimum required for success is six hits, which is not an unknown quantity since receiving the wage is established by reaching that minimum.