Simple calculation method of 7.8 * 5.6 + 3.9 * 8.8

Simple calculation method of 7.8 * 5.6 + 3.9 * 8.8

7.8×5.6+3.9×8.8
=7.8×5.6+3.9×（7.8+1）
=7.8×5.6+3.9×7.8+3.9
=7.8×（5.6+3.9）+7.8×0.5
=7.8×（9.5+0.5）
=7.8×10
=78

Simple calculation of 3.7 × 9.8

3.7x10-3.7x0.2= 37-0.74=36.26

7 out of 13 (14 out of 19-6 out of 13) help me!

7 out of 13 - (14 out of 19-6 out of 13)
= 7/13 + 6/13 -(14/19
= 1-14/19
= 5/19

2 / 3-7 / 12 + 9 / 20-11 / 30 + 13 / 42-15 / 56

2/3-7/12+9/20-11/30+13/42-15/56
=2/3-(1/3+1/4)+(1/4+1/5)-(1/5+1/6)+(1/6+1/7)-(1/7+1/8)
=2/3-1/3-1/8
=1/3-1/8
=5/24