How do you find the percentage increase between 1.55 and 2.38?

53.56% approximately.
The increase is $2.38 - 1.55 = 0.83$.
This increase is over $1.55$. Hence percentage increase is
$\frac{0.83}{1.55} \times 100 = \frac{83}{1.55} = 53.55$% approximately