io.write("Would you like to add, subtract, divide or multiply? \nAnswer either 'add' 'sub' 'div' or 'mul': ");

while answer1 ~= "add" and answer1 ~= "sub" and answer1 ~="div" and answer1 ~="mul" do
    answer1 = io.read();
        if answer1 ~= "add" and answer1 ~= "sub" and answer1 ~= "div" and answer1 ~= "mul" then
            io.write("Please choose a valid response: ");
        end

if answer1 == "add" then

    io.write("Number to add to: ");

    while num1 == nil do
        num1 = tonumber (io.read());
            if num1 == nil then
                io.write("Please input a number: ");
            end
    end

    io.write("Number to add by: ");

    while num2 == nil do
        num2 = tonumber (io.read());
            if num2 == nil then
                io.write("Please input a number: ");
    end
end
    io.write("Your answer is: " .. num1+num2 .. "\n");

elseif answer1 == "sub" then

    io.write("Number to be subtracted: ");
    while num1 == nil do
        num1 = tonumber (io.read());
            if num1 == nil then
                io.write("Please input a number: ");
            end
    end

    io.write("Number to subtract by: ");
    while num2 == nil do
        num2 = tonumber (io.read());
            if num2 == nil then
                io.write("Please input a number: ");
    end
end
    io.write("Your answer is: " .. num1-num2 .. "\n");

elseif answer1 == "div" then

    io.write("Number to be divided: ");
    while num1 == nil do
        num1 = tonumber (io.read());
            if num1 == nil then
                io.write("Please input a number: ");
            end
    end

    io.write("Number to divide by: ");
    while num2 == nil or num2 == 0 do
        num2 = tonumber (io.read());
            if num2 == nil or num2 == 0 then
                io.write("Please input a valid number: ");
    end
end
    io.write("Your answer is: " .. num1/num2 .. "\n");

elseif answer1 == "mul" then

    io.write("Number to be multiplied: ");
    while num1 == nil do
        num1 = tonumber (io.read());
            if num1 == nil then
                io.write("Please input a number: ");
            end
    end

    io.write("Number to multiply by: ");
    while num2 == nil do
        num2 = tonumber (io.read());
            if num2 == nil then
                io.write("Please input a number: ");
    end
end
    io.write("Your answer is: " .. num1*num2 .. "\n");

end

end