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?
Let's think: First find the total bug population after spraying: 80% * 400 bugs = 320 bugs. Then find the total number of bugs each spider eats: 12 spiders * 7 bugs / spider = 84 bugs. Then subtract that number from the number of bugs left after spraying to find final number of bugs: 320 bugs - 84 bugs = 236 bugs.. The answer is: 236.

A couple's wedding reception had 180 people. The bride and groom had each invited 20 couples from their side of the family. The remaining guests were friends. How many friends attended the reception?
Let's think: From the bride's side, 20 couples * 2 guests / couple = 40 guests attended. From the groom's side, 20 couples * 2 guests / couple = 40 guests attended. Therefore 180 guests - (40 guests + 40 guests) = 100 friends attended the reception.. The answer is: 100.

Building one birdhouse requires 7 planks and 20 nails. If 1 nail costs $0.05, and one plank costs $3, what is the cost, in dollars, to build 4 birdhouses?
Let's think: The cost of the planks for one birdhouse is 7 * 3 = $21. And the nails are a cost of 20 * 0.05 = $1 for each birdhouse. So to build one birdhouse one will need 21 + 1 = $22. So the cost of building 4 birdhouses is at 4 * 22 = $88.. The answer is: 88.

Johnny has been playing guitar for a while now. He practices the same amount each day. As of 20 days ago he had half as much practice as he has currently. How many days will pass before Johnny has 3 times as much practice as he does currently?
Let's think:
If Johnny had half as much practice as he currently has as of 20 days ago, this means he has 20 * 2 = 40 days worth of practice currently. Therefore, to have three times as much practice as he currently has, he'd need to practice for 40 * 3 = 120 days. Since he's already practiced for 40 days, this means he needs to practice or 120 - 40 = 80 more days.. The answer is: 80.