Step-by-step explanation:

Put numbers in order: 4,5,6,6,7,8,9,9,10,10,12,12,14,15
Cut the list into two equal parts: 4,5,6,6,7,8,9 and 9,10,10,12,12,14,15
Find middle numbers: 4,5,6,6,7,8,9 and 9,10,10,12,12,14,15
Lower quartile: 6
Upper quartile: 12
The difference of the values of the first and third quartiles of the data set is 12 - 6 = 6
1.5x will be the very answer
6 is 75% of 8. 6/8 can be simplified to 3/4, which is easier to see as 75%.