A fortieth is 1/40
In order to convert 2/8 to x/40, you will need to multiply both numerator and denominator by 5 (Since 40 / 8 = 5)
So
2/8 = 10/40
An alternative to solving this is to note that 2/8 itself can be reduced to 1/4, in which case changing to fortieths would require multiplying numerator and denominator by 10.