For the first problem, a "fifth" means divided by 5, so "four-fifths of b" means [tex](b/5)*4[/tex] or [tex]4b/5[/tex]
Subtract means to take away, so "6 subtracted from" means to "take away 6" or -6
As such, the answer to the problem would be [tex]4b/6[/tex]
The rest the problems are also just translating English to math.
