A box contains 15 cards numbered 1 to 15. Five cards are drawn at random one after the other with replacement. The numbers obtained from the cards drawn are noted. The probability that

a) Every number should be less than or equal to 7 and so  ${\left(\frac{7}{15}\right)}^5$
b) Every number should be less than or equal to 7 but all the numbers should not be less than 7 so that we get at least one 7 hence ${\left(\frac{7}{15}\right)}^5-{\left(\frac{6}{15}\right)}^5$similar method for c & d