Finding voltages in a circuit with current source
You can calculate the current through the R2/R3/R4 branch using the current divider rule. Knowing the equivalent resistance of the R2/R3/R4 combination is 50 ohms,

$$I_{234} = I_1\frac{1/50}{1/50 + 1/75}$$

$$I_{234} = (25)\frac{75}{75 + 50}$$

$$I_{234} = (25)\frac{75}{125}$$

Which is the form given by your book.