You can do this in your head because if it takes an hour for
the potato to heat up from 20° to 140°, which is an increase 
of 120°, then to heat up from 20° to only 50°, an increase of 30°
which happens to be 1/4 of 120°, it will obviously take it only
1/4 as long or 1/4 of an hour or 15 minutes.

But here is how to do it using algebra:

When time = 0 minutes, the temperature of potato is 20°
when time = 60 minutes, the temperature of the potato is 140°
When time = ? minutes, the temperature of the potato is 50°.

This problem reduces to:

Find the equation of the line through the points 

(x1,y1) = (0,20) and (x2,y2) = (60,140),

Then find x when y = 50

 







So the slope is m = 2

Now we find the equation of the line by substituting 

m = 2 and (x1,y1) = (0,20)

into the point-slope formula for the equation of a line:

Don't substitute 

y - y1 = m(x - x1)

y - 20 = 2(x - 0)

y - 20 = 2x

     y = 2x + 20

Now we want to find x when y = 50
So we substitute 50 for y and solve for x:

    50 = 2x + 20
    30 = 2x
    15 = x

So it will take 15 minutes.

Edwin