Question

how did newton know that 1%of 80 is 0.8

Answer 1

So,

Newton knows that 1% of anything really just means moving the decimal point two places to the left.

Examples:
1% of 10 = 0.1

1% of 389,092,192 = 3,890,921.92

1% of 3.1415926 = 0.031415926

Knowing this, Newton figures that all he has to do is move the decimal place two places to the left.

1% of 80 = 0.8