Subtract 0.9 Using a Visual Grid Model: Decimal Subtraction with Tenths

Let's solve the subtraction problem step by step:

• Step 1: Align the decimal numbers. The subtraction is $1.8 - 0.9$. Both numbers have their decimal points aligned.
• Step 2: Subtract the numbers starting from the tenths place: $8$ (in $1.8$) minus $9$ (in $0.9$). This requires borrowing.
• Step 3: Regroup, take $1$ from the units place of $1.8$, which then becomes $0.8 + 10$ (or $10$ tenths), thus $18$ tenths. Subtract $9$ tenths from $18$ tenths, resulting in $9$ tenths.
• Step 4: The units digit of $1.8$ now, after borrowing, is $0$. There is nothing left to subtract, so the remaining digit in the unit place remains $0$.
• Step 5: Therefore, the answer obtained is $0.9$.

Thus, the solution to the problem is $\mathbf{0.9}$.