How to read voltage using ADS1115
As KIIV mentioned you need to do the math:

Use this online voltage divider calculator to calculate your maximum input.


Keep in mind you shouldn't exceed Arduino 5V.
Use a multimeter to measure the resistors actual resistance.
Do the math for your example assuming R1 is exactly 100K and R2 10K you have to multiple the voltage by 11.
remember any error is going to multiple by 11 too, so make sure the first reading is accurate/stable.


ADS1115 is actually 15bit which will give you resolution of 32768, with default PGA setting you get:
6.144 / 32768 = 0.0001875V per bit.

Voltage = (your ADC reading) * 0.0001875V;
Voltage = Voltage * 11;


Take a look at This question/answer to see how I did reduced Arduino ADC noise, some steps can be done for ADS1115 as well.