How is using mental math to add with decimals like using mental math to add whole numbers

Well, they are the same kind of because if you say 20.4+7.3 lets round it 20.4 would round to 20 and 7.3 would round to 7 so 20+7=27.

Rounding whole numbers would be 25+ 62, 25 would round to 30 and 62 would round to 60, so 30+60= 90  Exact 25+62= 87, which is really close. :) They are really the same because you are just rounding and you can round by tenths while you can't for whole number. Whole number are much easier.

Hope I helped :-)