2021 AMC 10 B Fall

Complete problem set with solutions and individual problem pages

Problem 4 Easy

At noon on a certain day, Minneapolis is N degrees warmer than St. Louis. At 4: 00 the temperature in Minneapolis has fallen by 5 degrees while the temperature in St. Louis has risen by 3 degrees, at which time the temperatures in the two cities differ by 2 degrees. What is the product of all possible values of N ?(2021 AMC Fall 10B, Question #4)

  • A.

    10

  • B.

    30

  • C.

    60

  • D.

    100

  • E.

    120

Answer:C

Solution 1:

At noon on a certain day, let M and L be the temperatures (in degrees) in Minneapolis and St. Louis, respectively. It follows that M=L+N. At 4: 00, we get \begin{aligned} |(M-5)-(L+3)| &=2 \\ |M-L-8| &=2 \\ |N-8| &=2 \end{aligned} We have two cases: 1. If N-8=2, then N=10. 2. If N-8=-2, then N=6. Together, the product of all possible values of N is 10 \cdot 6=\mathbf{( C )} 60.

Solution 2:

At noon on a certain day, the difference of temperatures in Minneapolis and St. Louis is N degrees.

At 4: 00, the difference of temperatures in Minneapolis and St. Louis is N-8 degrees. It follows that |N-8|=2 We continue with the casework in Solution 1 to get the answer (\mathbf{C}) 60.