2/5+1/4÷5/6
=2/5+1/4×6/5
=4/10+3/10
=7/10
设这个数是x
则75%x-4.8=5.1
0.75x=5.1+4.8=9.9
x=9.9÷0.75=13.2
答:这个数是13.2