The formula is a permutation in which n represents the sample points in the set, while r represents the number of sample points in each permutation.
so 21 - 13, the money will be invested for 8 years.

Um... it'll take each pipe 80 mins, bcz if together they take that long, then to find the pipes time separately, you just multiply it by two to show it doing the work the other pipe did as well. Is that what you wanted??
Answer:
Table
Step-by-step explanation:
A table is an arrangement of data in rows and columns, or possibly in a more complex structure.