percent means parts out of 100
0.5%=0.5/100=0.05/10=0.005/1=0.005
'of' means multiply
'is' means equals or '=' sign
so
0.5% of what number is 3 translates to
0.005 times what=3
divide both sides by 0.005
0.005/0.005 times what=3/0.005
1 times what=600
what=600
the number is 600
