If I'm interpreting this question correctly, then you want to know the tax per dollar. In order to find that, you need to find the percentage of $420 that equals $60. So,
420x = 60
x = 60/420
x = 0.1429 (rounded)
So, the tax rate would be this value multiplied by 100%.
0.1429 x 100% = 14.29%