what about using 20-30-40-5
Step-by-step explanation:
the sequence is that you are skipping ten to get the second nb which is 30 and keep on going on to get 50
One of the product rules for exponents was used.

Or, you can consider that the definition of an exponent and the associative and commutative properties of multiplication were used.
8.5x2=17
20.6-17=3.6
3.6-1.37=1.23
1.23-0.75=0.48
0.48/4=0.12
the spokes cost $0.12 each