13/20
make the common denominator 20
2/5 = 8/20
1/4 = 5/20
8/20 + 5/20 = 13/20
Answer:

Step-by-step explanation:
If t is the number of days since Sofia took the medication of 10 mg, then the amount of medication in her system after t days will be
.
So, the concentration of medicine in blood decreases by a factor of one half every day.
If we count the number of days from today and Lexi takes 10 mg of the same medicine four days later then the amount of medicine left on her body after t days will be
. (Answer)
Answer:
Step-by-step explanation:

We use P = i•e^rt for exponential population growth, where P = end population, i = initial population, r = rate, and t = time
P = 2•i = 2•15 = 30, so 30 = 15 [e^(r•1)],
or 30/15 = 2 = e^(r)
ln 2 = ln (e^r)
.693 = r•(ln e), ln e = 1, so r = .693
Now that we have our doubling rate of .693, we can use that r and our t as the 12th hour is t=11, because there are 11 more hours at the end of that first hour
So our initial population is again 15, and P = i•e^rt
P = 15•e^(.693×11) = 15•e^(7.624)
P = 15•2046.94 = 30,704
C. 20x = 200 because an equation always has an equal sign and other three are expressions because it doesn't have any signs, hoped this helped:)