Mental Arithmetic

Yesterday we were trying to calculate the salary to offer to someone who was going to work 25 hours or 62.5%.

Person Y
I've got a calculator on my phone.

St's head
That's five eighths.

Person Y
Now where did I put it? I turned it off for the interview. What was the number again?

St's head
Divide by 8, divide by 2, move decimal point.

Person X
It's going to be about £guess.

St
(After scribbling on the back of an envelope) The answer is... ( a precise number a little less than the guess)

Person X
How did you do that? Did you divide by 100 and multiply by 65 that quickly?

Person Z
62.5.

St
It's five eighths.

Person X
Oh, I was doing 65.

Person Y
The answer is... ( a precise number a little less than the guess, same as the previous one)

Does anyone know how to do mental arithmetic anymore? I wonder if some of the brain power that was once utilised doing number work is now side-tracked to dealing with the question, 'Which of my portable devices has a calculator app and where did I put it?'

I realise I am sounding a bit old. I'll be complaining that young people mumble next.