Why is the metric (si) system called a decimal system of measurement

The metric system is the oldest name for the international system of units. SI unit or the international systems of units are based on seven basic units; the meter, kilogram, second, ampere, Kelvin, candela and mole. All of these basic units are divided into multiples by a power of ten. For example in meters, 1 meter is equal to: 1000 millimeter, 100 centimeter, 10 decimeter, 0.1 decameter, 0.01 hectometer, 0.001 kilometer and so on and so forth.

2fe + 3h2so4 -> fe2(so4)3 + 3h2

it is necessary to balance equations in order to get the proper amount of reactants as well as products in order not to make a miscalculation.

you cannot change the subscripts as that would change the substance itself. for instance, diatomic elements such as h2 only have the subscript two when a gas. another example would be so4, if you were to change the subscript, the element would no longer be sulfate.

we change the coefficients instead as that only changes the number of molecules rather than the substance itself.

hope this !

since electrons are negative the answer is neagtaive