Plan the calculation
0.6˙+0.3˙ Convert each recurring decimal to a fraction, then add. Watch for a surprise at the end.
Convert 0.6666...: name it x
x=0.6666666… Let x=0.6666....
Multiply x by 10
10x=6.6˙ One repeating digit, so multiply by 10.
Subtract and solve for x
9x=6⇒x=32 Subtracting clears the tail: 9x=6, so x=2/3.
Convert 0.3333...: name it y
y=0.3333333… Let y=0.3333....
Multiply y by 10
10y=3.3˙ One repeating digit, so multiply by 10.
Subtract and solve for y
9y=3⇒y=31 Subtracting clears the tail: 9y=3, so y=1/3.
Add the two fractions
32+31 x=2/3 and y=1/3, both already in thirds.
Add the numerators
32+1=33 Same denominator: 2+1=3 over 3.
Simplify to a whole number
Three thirds make one whole. The two recurring decimals add to an exact whole number.
Check directly by adding digits
0.6˙+0.3˙=0.9˙ Adding the recurring decimals digit by digit gives 0.9999… = 0.9999....
Recall that 0.9 recurring = 1
0.9˙=1 0.9999... is exactly 1 (not just close to it), so both methods agree.
Confirm the two methods match
32+31=1=0.9˙ The fraction method and the digit method give the same answer of 1.
Sense check with decimals
0.666…+0.333…≈1 Roughly 0.666+0.333=0.999…, which is 1. It looks surprising that two never-ending decimals add to a tidy whole number. ✓
State the answer
So 0.6666... + 0.3333... = 1 exactly, because 0.9999... = 1.