Answer:
35
Step-by-step explanation:
To find one part of five, divide 21 by 3, which gives you 7. Now multiply that 7 by five (to give you the full five parts) and you get 35.

In other words, how many ways are there to choose

objects from a total of

objects? Just one; take all of them at the same time.
X2 - 25 = 0
Add 25 on each side.
x2 = 25
Divide by 2 on each side.
x = 12.5
15 and one half is 31/2. 3 and seven eighths is 31/8. 31/2 also equals 124/8. Then the answer is 124/8 - 31/8 is 91/8. Hope that helped.