How to write the question 13 on page 77 of the first volume of junior one mathematics?

Solution: let the second inner angle be x, according to the meaning of the question:

Equation relation: first internal angle degree+second internal angle degree+third internal angle degree = total degree;

Therefore, the first internal angle is (3x) degrees;

The third inner angle is (x+ 15) degrees;

According to the equation, the formula is:

3x+x+x+ 15= 180

5x+ 15 = 180

5x = 180- 15

x = 165÷5

x =33

The second internal angle is 33 degrees,

The first internal angle is 33*3=99 degrees;

The third inner angle is 33+ 15=48 degrees;

A: The first internal angle is 99, the second internal angle is 33 and the third internal angle is 48.

I hope it can be adopted. Thank you.