Answer:

-10416
Step-by-step explanation:
a = 4
r = -5
geometric series: 
Determine which term = -12500:








Using geometric sum series formula: 
Therefore, 
Answer:
12 minutes
Step-by-step explanation:
Since it takes 4 minutes to fill the bathtub with both taps on and stopper is in then for each minute the 2 taps can fill up to a quarter (1/4) of the tub:
Similarly, it takes 6 minutes to drain the tub with both taps off and the stopper is out. This means for each minute the stopper can drain one sixth (1/6) of the tub.
Therefore, when both taps are on and the stopper is out, then for each minutes, the amount of water remaining in the tub is the water flowing in minus the water draining out

So for each minute, one twelveth of the tub is filled, for the tub to be completely filled in, it would take 12 minutes
Answer: In the last steps, instead of multiply each side by 3, he has to divide each side by 3.
Step-by-step explanation:
Hi, Tobin makes the mistake in the last steps.
3(x-1.25) = 11.25
3x-3.75= 11.25
3x-3.75+3.75= 11.25+3.75
3x = 15
Until this step is the work ok.
Now, instead of multiply each side by 3, he has to divide each side by 3:
So:
3x /3 = 15/3
x = 5
Feel free to ask for more if needed or if you did not understand something.
2 or 3 I would say... hope that helps : )