(75- 15×2)÷(7-2)=9 (branch)

Equation:

Solution: If there are X pens, there are (15-x) pencils.

7x+2( 15-x)=75

7x+30-2x=75

7x-2x+30=75

5x+30-30=75-30

5x=45

x=9

Xiaoming bought nine pens.