if we want to calculate the full prize we need to calculate how much the tax was:
420 17\(\frac{1}{2}\)%=420\(\frac{175}{1000}\)=420\(\frac{7}{40}\)=21\(\frac{7}{2}\)=73.5
so this is how much the tax was.
and the full prize is the cost of the ring and the tax: 420+73.5=493.5
so that's how much the buyer will pay
