Answer:
The first angle is
.
Explanation:
Let the measure of the second angle be represented by
.
Thus, the first angle =

Third angle =

Sum of angles in a triangle =

So that;
+
+ 90 =

+
+
=

=
-

=

divide both sides by 4 to have;
=

The first angle =

= ( 3 x 20 + 10)
= 60 + 10
=

Therefore, the first angle is
.