First you have all ten books, but then every time after, you loose one because you can't put the same book in the group twice.
10 * 9 * 8 * 7 = 5040 groups
X = 180 - 150 = 30°
z = 180 - 100 = 80°
y = 180 - 30 - 80 = 70°
Answer: x = 30, y = 70, z = 80
1600<2570-125.5x<2000 subtract 2570 from all terms...
-970<-125.5x<-570 divide all terms by -125.5 (and reverse signs because of division by a negative!)
7.73>x>4.54 and x is months since January, and since months can only be integers...
x=[5,7]
So January + 5, 6, and 7 respectively are the three months that satisfy the equation...
June, July, and August.
Answer:
Look at the picture. The answer and the steps are all in it.