30X+45(X+4)= 1755
30X+45X+ 180 = 1755
75X= 1575
X=2 1
X+4=2 1+4=25
2①: If you buy X pens, then the number of brushes is105-X.
2 1X+25( 105-X)=2447
2 1X+2625-25X=2447
4X= 178
X=44.5
It is impossible to buy 0.5 pieces, so Mr. Wang said that it was wrong for him to use this money to buy only these two pieces.
② According to the calculation in ①, in order to make the number of purchases an integer, 2 yuan or 6 yuan (an integer less than 10) must be subtracted from the total payment.
If you subtract 2,
2 1X+25( 105-X)=2445
2 1X+2625-25X=2445
4X= 180
X=45
If the number of pens purchased is 45, the number of brushes purchased is 60,21x45+25x60 = 2445 yuan.
If you subtract 6,
2 1X+25( 105-X)= 244 1
2 1X+2625-25X=244 1
4X= 184
X=46
If the number of pens purchased is 46, then the number of brushes purchased is 59,21x46+25x59 = 2441yuan.
So the unit price of the pen may be 2 yuan or 6 yuan.