Not all info is here. what is the original cost and %off?
I guess that u = μ & that a=σ. If so:
μ =100 & that σ =20 & x=20
Z score = (x-μ) / σ ==> Z score = (90-100)/20 ==> Z = - 0.5
Divide it by 4/4. You should get 7/8. 7/8 is the same as 28/32. 28/4=7, 32/4=8. Therefore, you can divide it by 4/4.
