Fast is etymologically related to Hard. Fast is the opposite of Weak. Weak is a synonym of Accessible. 
Equal is related to Sum. Sum is related to Addition. Addition is the opposite of Reduction. Reduction is a synonym of Discount. 
Hard <--EtymologicallyRelatedTo-- Fast <--Antonym--> Weak <--Synonym--> Accessible 
Equal <--RelatedTo--> Sum <--RelatedTo--> Addition <--Antonym--> Reduction <--Synonym--> Discount 