Answer:
50
Step-by-step explanation:
Using the experimental probability, first we have to find total amount paint then to find the amount of good quality forever paint we have to subtract the amounts of other paints from total amount.
Let x represents the amount of good quality forever paint sold.
Total amount of paint
...(i)
Experimental probability of paint sold will be better quality durable paint is


It is given that experimental probability of paint sold will be better quality durable paint is 


...(ii)
From (i) and (ii), we get



Therefore, 50 gallons good quality forever paint is sold.