The compatible numbers you pick are entirely up to you and they must be easy to manipulate mentally. Pick 45 rather than 40 (45 is nearer to 42.693 than 40) and divide by 5 to give you 9. There's no real right and wrong when choosing compatible numbers. Just get close and comfortable, and keep it simple. The actual answer is 9.0836 approx., so 9 is a good estimation.