Simple calculation of 0.99/4.12 Simple operation of 0.99 times 4.12``

Simple calculation of 0.99/4.12 Simple operation of 0.99 times 4.12``

0.99 times 4.12
1×4.12-0.0412
=4.12-0.0412
=4.11588

(1) Simple operation of 102 * 99

Two methods: 102 × 99 = (100 + 2) × 99 = 100 × 99 + 2 × 99 = 9900 + 198 = 10098 method 2: 102 × 99 = 102 × (100-1) = 102 × 100-102 × 1 = 10200-102 = 10098

Interesting simple calculation 1.9 + 19.9 + 1999.9 + 0.3
There must be an operation process!
thanks
I really don't know how to use simple calculation

=1.9+19.9+1999.9+0.1+0.1+0.1
=（1.9+0.1）+（19.9+0.1）+（1999.9+0.1）
=2+20+2000
=2022