Help with double and float

[Mizfizz]

whats the difference i know that 6.2 is a float and 99.99 is a double

but when i make a calculator that uses double

i can enter something like 6.2 and 7.2 to get an answer

but isnt it meant to only work for double numbers like 66.66

im a bit confused

ALSO

what is an expression can you give me a example in a program(please show a example and mark where the expression is and its meaning)

thx for listening i am a total beginner btw

also what is a namespace if you got time

[MatthewRock]

Double number isn't number with two digits after dot.
Float isn't a number that is floating anywhere.
For all your questions - simply try to google it("double" "double C++" "what is double" "what is namespace C++" etc.). You will find answers, most likely well-written.

Cheers!