math w/o calculator :
What's 18 * 25?
Rough ans is 20*25=500. Can we increase the accuracy?
Second method : round one of the numbers and check what percent of that number has been changed. And then increase or decrease the final result with that much percent.
So 18 rounded up to 20. Now 20*25=500 minus 10% = 450.
Real ans is : 450.
Why subtract 10%? Because when we round 18 up to 20 we basically increased it by 10%.
How do you know 500 less 10% = 450?
Subtracting 10% is easy. Shift the first digit right one step and subtract that number from the original. Thus 10% of 500 is 50.
Another one. What's 28*64?
Round 28 up to 30 that's about 5% increase. Multiply 30*64=1900.
Next less 5% from 1900. 1900-100=1800.
Real ans : 1792