You should know that $ in dart is a reserved keyword and if you want to use on it's own you must escape it using backslash "\". 

String concatenation Examples: 

A = "Hello " + "World"

A will show "Hello World"

String Interpolation: 

My_number = 10.00

inter = 'Your number is $MY_number'

if you have multiple operations in your string interpolation you need to use {}

inter_2 = 'Your number is ${My_number * 2}'

if you need to use the dollar sign you have the escape the dollar sign: 

inter_3 = 'Total amount is \$${My_number * 2}' 

\$ tells dart that I want to use the raw dollar sign and not the reserved keyword, the second dollar sign lets you inject your variable into the String.