How is minus 40 the same in F and C degrees?

Answer 1

That is because F = 9/5 C + 32

for F and C to be the same, F = C, solve:

F = C = 9/5 C +32

C -9/5 C = 32

-4/5C = 32

C = -5/4 (32) = -40

F = C = -40

Answer 2

Minus 40 degrees Fahrenheit is equivalent to minus 40 degrees Celsius. This is the only point where the two temperature scales intersect and have the same numerical value.