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Simplify the following:
2/5 y-9/10 y-4+7

Put each term in 2/5 y-9/10 y-4+7 over the common denominator 10: 2/5 y-9/10 y-4+7  =  (4 y)/10-(9 y)/10-40/10+70/10:
(4 y)/10-(9 y)/10-40/10+70/10

(4 y)/10-(9 y)/10-40/10+70/10 = (4 y-9 y-40+70)/10:
(4 y-9 y-40+70)/10

Grouping like terms, 4 y-9 y-40+70 = (70-40)+(4 y-9 y):
((70-40)+(4 y-9 y))/10

4 y-9 y = -5 y:
(-5 y+(-40+70))/10

70-40 = 30:
(30-5 y)/10

Factor 5 out of 30-5 y:
5 (6-y)/10

5/10 = 5/(5×2) = 1/2:
Answer: | (6-y)/2