578x161= 420000Step-by-step explanation:578 -------100------> 600161---------100-------> 700600x700= 420000HOW TO ESTIMATEThere is a target which we want to round off to. There are general rule and by 10,100,1000 etcIf it is general rule our target is the ten CRORES,crores,ten lakhs, lakhs, ten thousands, thousands, hundreds and tens.It can be more than ten croreeg:- 12678 so here we will estimate ten lakhs like wise in 4580 thousands, 78099 ten thousands, 67 tens etcfor 10 we will only round 10= 90090 here we will target 9 at 10s place like wise 56490- 9, 80908- 0 in tensthen when we GET the target we see to its right DIGIT. There are two groups 1 st group( 0,1,2,3,4) 2nd group( 5,6,7,8,9).So if it is 1st group, in the target we add 0 and if it is 2nd group we add to the target 1 and target digit and the nearest right digit except these both the digits in the right we will put 0 and in left same number.For your more understanding u can watch this videoMath Antics roundingAs I am not able to give u the linkHOPE IT HELPS UPLEASE MARK IT AS BRAINLIEST