10 is close to 12 and 340 is close to 336, so dividing 340 by 10 just means removing the zero. This gives an estimate of 34. We can also recognise that 336 is only a bit less than 360, and we know that 12 divides into this 30 times. So we have two numbers 34 and 30 as estimates of the true division which is 28.