You could do it as if they were fractions
.4 equals 4/10 which equals 2/5
.25 equals 25/100 which equals 1/4
2/5 times 1/4 equals 2/20
2/20 equals 1/10
1/10 equals .1
.1 is correct
You could also multiply decimals like this:
1.Multiply them together like full numbers
4 times 25 equals 100
2.Move the decimal in three times because the decimal .4 is moved in once and .25 is moved in twice so when you add them you get three
The answer would be .1 or .100 (it doesn't make a difference because you cross out any zeros at the end of a decimal Ex: .12300 equals .123 but .123001 stays .123001)