帮我解几道数学的递等式巧算,蟹蟹!要巧算的哦~

1、15.72-4.38÷2.4
=15.72-4.38÷0.3÷8
=15.72-14.6÷8
=15.72-1.7
=14.02
2、7.2×2.02÷2.4
=2.4×3×2.02÷2.4
=3×2.02×(2.4÷2.4)
=6.06×1
=6.06
3、9.8×25
=(10-0.2)×25
=10×25-0.2×25
=250-5
=245