Use mental math to estimate the unknown number 4210 divided by 6

6 times 7 42 plus 4210

6 divides into 42 producing a quotient of 7 and no remainder. All that's left is to divide 6 into 10. That produces 1 with a remainder of 4.

So you could say immediately that 4210 divided by 6 is approx. 71.