Differential Equation 02

The population in a certain city is 50,000 and is increasing at a rate numerically equal to the square root of the population. What will be the population 20 years hence?

• 57,980

• 67,687

• 54,571

• 55,987

The rate of population growth of a country is proportional to the number of inhabitants. If a population of a country now is 40 million and expected to double in 25 years, how many years will the population be 3 times the present?

• 39.62 years

• 28.75 years

• 25.45 years

• 43.34 years

The substance decomposed at a rate proportional to the amount present. What is the half-life of the substance if 10% decomposes in 100 years?

• 568 years

• 465 years

• 658 years

• 586 years

A certain radioactive substance has a half-life of 38 hr. Find how long it takes for 90% of the radioactivity to be dissipated.

• 145.7 hr

• 135.7 hr

• 126.5 hr

• 130.5 hr

A thermometer reading 18F is brought into a room where the temperature is 70F; a minute later, the thermometer reading is 31F. Determine the temperature reading 5 minutes after the thermometer is first brought into the room.

• 57.67F

• 55.35F

• 58.67F

• 60.57F