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.