If the solution is as follows:

The third line is the key, and the first and second lines are constants.

(The first line produces 4 batches of products every week, and the second line produces 6 batches of products every week. )

Set up three production lines to produce one batch of X products and two batches of Y products every week.

3x+2y= 18

The profit is:

4 * 3+5 * 6+3x+5y = 42+3x+2y+3y = 60+3y

The maximum value is: y is the maximum value.

That is, the third line completely produces product II, 18/2=9.

That is, the maximum profit is 60+3*9=87