<span>11.27%
There are 2 things to calculate first. That being "over how many years?" and "by what factor?"
So years first.
2013 - 1938 = 75
And now the factor
300000 / 100 = 3000
So the painting is worth 3000 times what it originally cost over a period of 75 years. And now all we need to do is calculate some number R such that R raised to the 75th power equals 3000. That is easily done with logarithms. We want the 75th root of 3000. So take the logarithm of 3000, divide by 75, and then take the anti-log. So
R = 10^(log(3000)/75) - 1
R = 10^(3.477121255/75) - 1
R = 10^0.046361617 - 1
R = 1.1126578 - 1
R = 0.1126578
So the painting increased in value by 11.27% each year.</span>