Just solve the equation! if the dot means multiply, then
35 - (5 x 2) + (16 / 4) because of BIDMAS or PEDMAS or whatever abbreviation you use, where the order is Brackets Indices Division Multiplication Addition and Subtraction.
You can try finding numbers that are close to what you think you can use for exaple lets say we want to find the answer to 54x34 you could multiply 50x30 to get an estimated answer
