A shopkeeper earns a profit of 2 by selling one marker pen and incurs a loss of 1 per eraser.

a) In January, he experienced a loss of 5 after selling 40 marker pens and some erasers. How many erasers did he sell during that month?

b) In the following month, there was neither a profit nor a loss. If he sold 65 marker pens, how many erasers did he sell?