3.9+3.99+3.999+3.9999=4×4-0.1-0.01-0.001-0.0001=16-0.1111=15.8889
3.9+3.99+3.999+3.9999=4-0.1+4-0.01+4-0.001+4-0.0001=(4+4+4+4)-(0.1+0.01+0.001+0.0001)=16-0.1111=15.8889
4-0.1+4-0.01+4-0.001+4-0.0001=12-0.1111=11.8889