Yea, even as someone who can do Hard Maths, mental arithmetic is not a Thing I can Do, nor need to do.
The only feat of mental arithmetic I can do at party-grade is the conversion of miles to Km.
I can mentally do the x1.6 in my head, and produce an answer to 1 decimal place.
There is a trick, of course.
Instead of trying to multiply by 1.6, multiply by 16 then divide by 10.
Both of these are much simpler operations.
Multiplying by 16 is simply doubling four times.
That's not so hard mentally.
Then, dividing by 10 is just a decimal point shift.
So, for example, 12 miles:
Double four times: 12 > 24, 48, 96, 192
Divide by 10.
Answer is 19.2k