a.
Noah put marbles on a bag, each color marble represents that the battery of the toy died before 15 hours (red marbles) or that the battery lasted after 15 hours (green marbles)
There are 4 batteries on the toy, for each battery there are two possible outcomes, each one represented by a red and green marble.
So, for each battery, he has to put two marbles. There will be 4 red marbles and 4 green marbles in the bag.
b.
To estimate the probability that at least one battery will die within 15 hours, you have to calculate the expected value.