If three angles in a quadrilateral are in the ratio of 1:2:3 and the measure of the 4th angle is 90°, then find the measure of other angles.

Let's assume, 

$\angle \mathrm{A}=\mathrm{x}, \angle \mathrm{B}=2\mathrm{x},\angle \mathrm{C}=3\mathrm{x}$.

Given, the fourth angle $\angle \mathrm{D}=90°$ .

According to the angle sum property of a quadrilateral $\angle \mathrm{A}+\angle \mathrm{B}+\angle \mathrm{C}+\angle \mathrm{D}=360°$

$\mathrm{x}+2\mathrm{x}+3\mathrm{x}+90°=360°$

$6\mathrm{x}=360°-90°$

$6\mathrm{x}=270°$

$\mathrm{x}=\frac{270°}{6}$

$\mathrm{x}=45°$

$\angle \mathrm{A}=1\mathrm{x}=1\times 45°=45°$ 

$\angle \mathrm{B}=2\mathrm{x} =2\times 45°=90°$

$\angle \mathrm{C}=3\mathrm{x}=3\times 45°=135°$

Therefore, the measure of other angles$\angle \mathrm{A}=45°, \angle \mathrm{B}=90°,\angle \mathrm{C}=135°$.