7. Given that each carbon pen costs 1 yuan 8 jiao, each notebook costs 3 yuan 5 jiao, and each stationery box costs 4 yuan 2 jiao, Jingjing bought these three stationery items and spent exactly 20 yuan. How many notebooks did she buy? $\qquad$
【Analysis】Assuming the purchase of only one type of item, calculate the maximum number that can be bought. Within this range, decompose 20 into the sum of several numbers' products.

【Solution】Solution: $20 \div 1.8 \approx 11$ (pens)
$$
\begin{array}{l}
20 \div 3.5 \approx 5 \text { (notebooks) } \\
20 \div 4.2 \approx 4 \text { (pencil cases) }
\end{array}
$$

Therefore, the maximum number of pencil cases that can be bought is 4, the maximum number of carbon pens that can be bought is 11, and the maximum number of notebooks that can be bought is 5. Observing the prices of the three stationery items, we find that the quantities of items priced at 1.8 and 4.2 should be equal, and the quantity of items priced at 3.5 should be even, $1.8+4.2=6$,

When the number of notebooks is 2, $20-3.5 \times 2=13, 13$ is not a multiple of 6, so it does not meet the condition;
When the number of notebooks is 4, $20-3.5 \times 4=6, 6 \div 6=1$,
Therefore, 20 is exactly spent on 1 pencil case, 1 carbon pen, and 4 notebooks;
Answer: She bought 4 notebooks.
The answer is: 4.