Acertain kind of animal weighs about 75 pounds at birth and gains about 2 pounds per day for the first few weeks. determine those days for which the animal's weight is more than 125 pounds. the animal's weight is more than 125 pounds when the animal is more than days old.

25

Let w represent the weight of the animal in pounds, and d the age in days. The problem statement tells us ...

... w = 75 +2d

We want to find d when w > 125.

... 75 +2d > 125 . . . . . substitute the above expression for w

... 2d > 50 . . . . . . . . . subtract 75

... d > 25 . . . . . . . . . . . divide by 2

The animal will weigh more than 125 pounds when it is more than 25 days old.

x = the number of minutes of calls

y = the total cost

Plan A:

y = 0.07x + 15      (costs $0.07 per minute of calls(x) plus a fee of $15)

Plan B:

y = 0.09x       (costs $0.09 per minute of calls(x) and has no initial fee)

Since you want to find when they cost the same for the same amount of minutes, you can set the equations equal to each other because they have the same cost

y = y

0.07x + 15 = 0.09x         To find x, isolate/get the variable "x" by itself in the equation. Subtract 0.07x on both sides to get "x" on one side

0.07x - 0.07x + 15 = 0.09x - 0.07x

15 = 0.02x        Divide 0.02 on both sides to get "x" by itself

750 = x

Now that you know x = 750, you can substitute/plug it into either equation to find y.

y = 0.09x    Plug in 750 into "x" since x = 750

y = 0.09(750)

y = 67.5  

y = 0.07x + 15        (proves they will cost the same)

y = 0.07(750) + 15

y = 52.5 + 15

y = 67.5

x = 750 minutes

y = $67.50

So both plans will have the same amount of minutes of 750 at the same cost of $67.50