Here's my code:

math.config({number: 'BigNumber', precision: 64})
math.eval('80 / day * 5 days')

I'm expecting the result '400'.

It works fine without configuring with BigNumber, but I do need to use BigNumber on my math object in general, as high precision might be important for the calculation and I don't know the user's expression ahead of time. I've tried BigNumbering parts of the expression, but haven't had any luck.