If I score 2/5 for the quiz and it worth 10% of my second exam. How many % needed to get 75%?

quiz is worth 10% and you got a 2/5 which is 40%. so we take 40% of 10% is 4% that you received so we take that away from the total (10%-4%) giving us 6% off of whatever you get on the exam. so now it's a matter of adding 6% to the wanted score of 75% so 75+6=81%....

in short
You would need a score an 81% on the exam because you did not do well on the quiz taking 6% from the total.
your final grade for the exam of 81%, minus the 6% loss from quiz will be 75%

Study harder next time.