To estimate the difference \( 86.9 - 18 \), we can round the numbers to the nearest whole numbers:
- Round \( 86.9 \) to \( 87 \).
- Round \( 18 \) to \( 18 \) (it remains the same).
Now, we can perform the subtraction:
\[ 87 - 18 = 69 \]
So, the estimated difference is approximately \( 69 \).