Well, first, let's solve the question for y so you don't have to keep doing that over and over. I'll just do essentially the same work you did, but on symbols:
(an + xy) / (n + x) = b
(an + xy) = b * (n + x)
xy = bn + bx - an
xy = n(b - a) + bx
y = (n(b - a) + bx)/x
y = n(b - a)/x + b
Now, for your given numbers, this is
y = 141(0.75 - 0.6176)/68 + 0.75 = 1.0245
y = 141(0.75 - 0.6176)/85 + 0.75 = 0.9696
Reversing this, if we take y as known and x unknown, we can solve my equation for x (a little easier than solving the original for x, because x appeared twice):
y = n(b - a)/x + b
y - b = n(b - a)/x
x(y - b) = n(b - a)
x = n(b - a)/(y - b)
For your numbers,
x = 141(0.75 - 0.6176)/(0.8 - 0.75) = 373.4
Yes, it can take a lot of time to increase an average! If you examine the formulas closely, you can see why. The "distance" you have to go (b-a) is almost three times the "distance" you've come (y-b), so it will take almost three times as long.