SOLUTION: The average temperature from Monday to Thursday is 48 degrees and from Tuesday to Friday is 52 degrees. If the temperature on Monday is 42 degrees, what was it on Friday

Let M be the temperature on Monday,
    T -------- '' --------- Tuesday
    W -------- '' --------- Wednesday
    H -------- '' --------- Thursday
    F -------- '' --------- Friday


You are given 

 = 48,  which implies  M + T + W + H = 4*48 = 192.   (1)


You also are given 

 = 52,  which implies  T + W + H + F = 4*52 = 208.   (2)


Subtract equation (1) from equation (2). You will get

F - M = 208 - 192,  or, which is the same,  F - M = 16.                     (3)


Now substitute  M = 42  into equation (3).  You will get

F - 42 = 16,  which implies  F = 16 + 42 = 58.


Answer.  The Friday temperature is 58 degrees Fahrenheit.