Hello there. I honestly wouldn't worry about it. My brother got a D in IGCSE maths, and then did GCSE maths as a retake and got a B (so maths isn't his strong point). He applied for business at a number of good unis (Manchester, Bath, Sheffield) got into all of them and went to Sheffield, where he graduated with a high 2.1. For businesss it really doesn't seem that maths is all that important. As far as an MBA is concerned, I'm not sure. I can't imagine you need to be amazing at maths, with calculators and all!