Question 566122
We have 3 (f for first, s for second, t for third) variables and 3 equations:
the measure of the second angle is 10 degrees more than three times the first.
s=3f+10
the third angle measure is equal to the sum of the measures of the other two:
t=f+s
And we know that interior angles in triangles add up to 180:
f+s+t=180
We can actually sub in for f+s and replace it with t since those are equal values

t+t=180
2t=180
t=90

Now we know that f+s=90
and that s=3f+10
so I can sub in 3f+10 for s
f+3f+10=90
4f+10=90
4f=80
f=20
Which makes the second 70