In how many ways can Susan arrange 7 books into 5 slots on her bookshelf?

2520

Step-by-step explanation:

This is permutation question

The formula for it:

N = b!/(b-s)!, where N- number of ways, b- number of books, s- number of slots

Finding the

N = 7!/(7-5)! = 7!/2! = 7*6*5*4*3 = 2520

The answer:

1. ∠3, ∠5 are supplementary and ∠4, ∠6 are supplementary exterior sides in opposite rays.
2. ∠5 = ∠6 and ∠1 = ∠2 cpcte
3. ∠3 = ∠4 reflexive
4. mn = mn supplements to = angles.
5. triangle mnq congruent to triangle mnp asa 6. mq = mp given
the statements given above are true, but for proving   mq = mp, it is sufficient to prove that the triangle mqp is an isosceles.
for that 
∠1 = ∠2, because the line mn bisects ∠m in two similar parts, and 
besides pq and mn are  perpendicular, then it is obvious that mqp is an isosceles triangle.