The rule starting with five. 5 marbles in first line.
Every consecutive line has a number that is four less than twice the previous line.
marbles in second line = 2(marbles in first line) - 4
So the number of marbles in second line = 2(5) -4 = 10-4 = 6
marbles in third line = 2(marbles in second line) - 4
So the number of marbles in second line = 2(6) -4 = 12-4 = 8
marbles in fourth line = 2(marbles in third line) - 4
So the number of marbles in fourth line = 2(8) -4 = 16-4 = 12
marbles in fifth line = 2(marbles in fourth line) - 4
So the number of marbles in fifth line = 2(12) -4 = 24-4 = 20
marbles in sixth line = 2(marbles in fifth line) - 4
So the number of marbles in sixth line = 2(20) -4 = 40-4 = 36
36 marbles must be in the sixth line.
