There are 10 x 3 = <<10*3=30>>30 green chairs.
The total number of blue and green chairs is 10 + 30 = <<10+30=40>>40.
So, there are 40 - 13 = <<40-13=27>>27 white chairs.
Therefore, there are 40 +27 = <<40+27=67>>67 chairs in a classroom.
#### 67