parbatya,
you rounded them numbers so i got diff. percentages when i calculated them (on my head). comes out to be 47.85% to be precise (with calc :P) so yeah that's one solution that you came up with.. Also n=4.0236 is the precise value in your solution (not 3.91), that will yield y=0.1 for x=0.4
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just to generalize this (for V2001 or for anyone who might be interested):

y=K.exp(nx)

the given condition, y=0.1 when x=40% can have multiple solutions for K and n.

putting these values in the above eqn:

n=[log(0.1)-log(K)]/0.4 (where log is a natural logarithm)

so based on your K, n is determined. To make sure it's a positive exponential curve (for positive increment), K<=0.1. Also to get real solution (not complex), K>0.

Hence 0<K<=0.1
Also, more the K, less will be your change in flow percentage (y%). Depending upon your requirement, K can be chosen and that will give n.