Agardener buys two kinds of fertilizer. fertilizer a contains 60% filler materials by weight and fertilizer b contains 40% filler materials by weight. together, the fertilizers bought by the gardener contain a total of 240 pounds of filler materials. which equation models this relationship, where x is the number of pounds of fertilizer a and y is the number of pounds of fertilizer b?
a) 0.4x + 0.6y = 240
b) 0.6x + 0.4y = 240
c) 40x + 60y = 240
d) 60x + 40y = 240