The problem with the above equation is that `amount` has a scale of 0. As we don't know the scale of the double (0.4) we use the scale of the amount. The resuls in 0.4 being rounded down to zero.
The correct way to handle this is to first convert the double to a Fixed instance with the desired scale.
Moneyprovides the following arithmetic operators:
-must be used with operands in same currency, otherwise
ArgumentErrorwill be thrown.
numas the second operand. Both operators use schoolbook rounding to round result up to a minorUnits of a currency.