Explain the meaning of the exponent 3 in the algebraic expression (x+y)^3

Since the exponent is 3, and in the parentheses is x+y, it's basically
(x+y)(x+y)(x+y)
You write out x+y 3 times since the exponent is 3.
If the exponent was 4, you would write out x+y 4 times and so on.
The 3 gives value to the x+y and how many to use or calculate before getting your answer.

Let x and y be the unit rates at which one large pump and one small pump works, respectively.

Two large/one small operate at a unit rate of

(1 pool)/(4 hours) = 0.25 pool/hour

so that

2x + y = 0.25

One large/three small operate at the same rate,

(1 pool)/(4 hours) = 0.25 pool/hour

x + 3y = 0.25

Solve for x and y. We have

y = 0.25 - 2x   ==>   x + 3 (0.25 - 2x) = 0.25

==>   x + 0.75 - 6x = 0.25

==>   5x = 0.5

==>   x = 0.1

==>   y = 0.25 - 2 (0.1) = 0.25 - 0.2 = 0.05

In other words, one large pump alone can fill a 1/10 of a pool in one hour, while one small pump alone can fill 1/20 of a pool in one hour.

Now, if you have four each of the large and small pumps, they will work at a rate of

4x + 4y = 4 (0.1) + 4 (0.05) = 0.6

meaning they can fill 3/5 of a pool in one hour. If it takes time t to fill one pool, we have

(3/5 pool/hour) (t hours) = 1 pool

==>   t = (1 pool) / (3/5 pool/hour) = 5/3 hours

So it would take 5/3 hours, or 100 minutes, for this arrangement of pumps to fill one pool.