Variable Scopes in Scala

Can anybody please help me with this exercise, I have gone through it a million times and I cannot understand how the output can be 42. I especially struggle with val bp. A walkthrough the code would be highly appreciated. Thanks a lot!

 object Scope {
    var a : Int = 1
    var b : Int = 3
    var c : Int = 0
  
    def m1(b : Int) : Int = {
       a = 5
       c += a + b
       b + 3
    }
    
    def m2(a : Int, d : Int) : Int = {
       b = c
       val bp = m1(a - d)
       c += b + bp
       b + c
    }

    def main(args : Array[String]) : Unit = {
       b = m1(a)
       a = m2(b,a)
       c += a
       println(c)
    }
}