a) x+2/15=27/45
x+2/15=9/15
x+2=9
x=7
b) a/b=0,4
a=0,4b
2a+3b/5a-b=2×0,4b+3b/5×0,4b-b=0,8+3b/2b-b=3,8b/b=3,8