A: "I remember that on a certain day in Winter both Celsius and Fahrenheit temperature readings showed the digit 5 as the units digit..."

B(after some mental calculation): "I'm certain it was not the coldest day..."

How cold was it?

As it happens, I recall that -40 C = -40 F
Since 5 Celsius degrees are the same size as 9 Fahrenheit degrees, I want to change -40 by 5N to get Celsius and by 9N to get Fahrenheit, but such that each result ends with 5.
N=5 does the trick.
temp = -40 + 25 = -15 C
temp = -40 + 45 =  +5 F

Other temps (not just winter in Manitoba temps) that end in 5 for both C and F are:
temp in degrees C = -15 + 50*k
temp in degrees F =    5 + 90*k
where k is any integer, positive or negative.

Edited on January 6, 2018, 12:32 pm

Posted by Larry on 2018-01-06 12:29:01