A bike has been marked down by 1/4 from the original price and the sales price is 128 dollar but I know how to do it i just don't get how we got 3/4 in the steps.

Thank you for the opportunity to help you with your question!

the price is marked down by 1/4, that means that the original price is reduce by 1/4, thus the new price is (1-1/4)=3/4 of the original price, let the original price is x, and we have

3/4x =128

x= 4*128/3 = 170.67

Please let me know if you need any clarification. I'm always happy to answer your questions.