How do you estimate decimals?

If you are rounding to the nearest tenth
Let's say round 20.15 to the nearest tenth. 1 is the tenth place. You have to look at the hundredths place. Since it is 5 and over You round up. If it is under 4 then you round down to it would be 20.2

1. Find the place value you want (the "rounding digit") and look at the digit just to the right of it.
2. If that digit is less than 5, do not change the rounding digit but drop all digits to the right of it.
3. If that digit is greater than or equal to 5, add one to the rounding digit and drop all digits to the right of it. 

Hope this helps

The equation is y=17(7)+600 and it equals 717