How do you do this properties and mental math to solve 275+180+120 =

Make 275 t0 200 and 180 to 100 and 120 to 100

200+100+100=300

now turn 75 to 70 and 80 will stay 80 and 20 will stay 20

70+80+20=170

now add the 5 to 170 = 175

now add 300 to 175 to get 475 which is ya answer