Answer:
9 hours
Step-by-step explanation:
According to Newton's laws of cooling
dT/dt = -k(T - A)
Let U = T - A
dU/dT = 1
dU = dT
dt/dt = -kU
dT = -kU(dt)
dU/U = -kdt
On integration
ln(U) = -kt + C
U = Ce^-kt
T - A = Ce^-kt
T(0) = 68
T(5) = 25
68 - 20 = Ce^-k(0)
C = 48
and
25 -20 = 48e^-k(5)
5 = 48e^-5k
e^-5k = 5/48
-5k = ln (5/48)
k = - ln(5/48) / 5
k = - 0.4524
T - 20 = 48e^-0.4524t
When T = 21
21 -20 = 48e^-0.4524t
1 = 48e^-0..4524t
e^-0.4524t = 1/48
-0.4524t = ln (1/48)
t = - ln(1/48) / 0.4524
t = 8.5570
t= 9 hours ( to the nearest hour)
Answer:Three times the difference of x and seven plus ten
Step-by-step explanation:
finding the product of 2 numbers means that you have to multiply those numbers
when multiplying fractions , numerators are multiplied by numerators and denominators by denominators
therefore product of 3/8 and 16 is 6
next we have to find 2/3 of 6
'of' operator means multiplication
answer is 4
