2022 AMC 8

Complete problem set with solutions and individual problem pages

Problem 9 Easy

A cup of boiling water (212^{\circ}\text{F}) is placed to cool in a room whose temperature remains constant at 68^{\circ}\text{F}. Suppose the difference between the water temperature and the room temperature is halved every 5 minutes. What is the water temperature, in degrees Fahrenheit, after 15 minutes?

  • A.

    77

  • B.

    86

  • C.

    92

  • D.

    98

  • E.

    104

Answer:B

Initially, the difference between the water temperature and the room temperature is 212-68=144 degrees Fahrenheit.

After 5 minutes, the difference between the temperatures is 144\div2=72 degrees Fahrenheit.

After 10 minutes, the difference between the temperatures is 72\div2=36 degrees Fahrenheit.

After 15 minutes, the difference between the temperatures is 36\div2=18 degrees Fahrenheit. At this point, the water temperature is 68+18=\boxed{\textbf{(B) } 86} degrees Fahrenheit.

 

Remark:

Alternatively, we can condense the solution above into the following equation:68+(212-68)\cdot\left(\frac12\right)^{\tfrac{15}{5}}=86.