Bill is trying to control the pests in his garden. Each spider he introduces eats 7 bugs, and each time he sprays the garden he reduces the total bug population to 80% of what it was previously. If the garden has 400 bugs to start, and Bill sprays once and introduces 12 spiders, how many bugs are left?
First find the total bug population after spraying: 80% * 400 bugs = <<80*.01*400=320>>320 bugs
Then find the total number of bugs each spider eats: 12 spiders * 7 bugs/spider = <<12*7=84>>84 bugs
Then subtract that number from the number of bugs left after spraying to find final number of bugs: 320 bugs - 84 bugs = <<320-84=236>>236 bugs
#### 236