[tzs]

For C to F you can often simplify the mental math by doing the multiply by doubling then taking off 10%.

E.g., to convert 31℃ to ℉: 31 x 2 = 62. Subtract 6.2 = 55.8. Add 32 = 87.8℉.

If you want to round the result to the nearest integer the subtract 10% step is a convenient place: 31 x 2 = 62. Subtract 6 (rounded 6.2) = 56. Add 32 = 88℉.

[SECProto]

Yes, doubling then subtracting 10% of the resultant works because that is the same as multiplying by 1.8 :)

When going the other way and dividing, I similarly find it mentally easier to multiply by 10/18 (rather than just divide by 1.8)