It could possibly be one.
Decimal points must be aligned with performing addition or subtraction.
The arithmetic operation that does <em>not</em> require decimal points be aligned is multiplication.
_____
For multiplication, you only need to know the number of digits to the right of the decimal point(s).
For long division, it is usually convenient to convert the divisor to an integer and adjust the decimal point of the dividend accordingly. Alternatively, it is necessary to know the number of digits in each number that are to the left of the decimal point.
By geometry, there is a relationship between the tangent PA and the segments PB and PE, namely
PA^2=PB*PE
so
PE=PA^2/PA=6^2/4=9
BE=PB-PE=9-4=5
4x+6=12
4x=12-6
4x=6
4x/4=6/4
X=1.5
Just plot multiples of 15 on the chart until you get to the 9th multiple
