Assume r= t not sure if you mistyped this.
t = 15 min
T(t) = 68 (0.5)^(15/10) = 24 Celsius
31 = (0.5)^(t/10) take ln
ln31 = (t/10) ln (0.5) + ln68
t = (ln32 - ln 68/ln0.5) * 10 = 11.3 min
Answer:
(C)Determine the principal square root of both sides of the equation.
Step-by-step explanation:
Given: Isosceles right triangle XYZ (45°–45°–90° triangle)
To Prove: In a 45°–45°–90° triangle, the hypotenuse is
times the length of each leg.
Proof:

Because triangle XYZ is a right triangle, the side lengths must satisfy the Pythagorean theorem, 
Since a=b in an isosceles triangle:

Therefore, the next step is to Determine the principal square root of both sides of the equation.