A) find, in ascending powers of X, the first 4 terms of the expansion (1+X)^7. B) Hence, find the coefficient of y^3 in the expansion of (1+y-y^2)^7.

Please help, this is mindbending. I understand (A) but not (B)​

You were told that the amount of time lapsed between consecutive trades on the new york stock exchange followed a normal distribution with a mean of 15 seconds. you were also told that the probability that the time lapsed between two consecutive trades to fall between 16 to 17 seconds was 13%. the probability that the time lapsed between two consecutive trades would fall below 13 seconds was 7%. what is the probability that the time lapsed between two consecutive trades will be between 14 and 15 seconds?