Writing equations

What equation matches this situation?
John wants to buy an MP3 Player that costs $30.98.  How much change did he receive if he gives the cashier $40.

• 40 + c = 30.98
• 30.98 + c = 40
• c - 40 = 30.98
• c - 30.98 = 40

Mei was going to sell all of her stamp collection to buy a video game.  After selling half of them she changed her mind.  She then bought nineteen more.  How many did she start with if she now has 45?

• 2/x + 19 = 90
• x/2 + 19 = 45
• 2x +19 = 109
• 19x + 2 = 71

Pizzazz Publications is having some books printed. The printer charges $800 plus $5 per book. What equation could help me find how many books can be printed for $4,000?

• 800 + 5x = 4000
• 800x + 5 = 4000
• 4000 + 5x= 800
• 800 + 5 = 4000

The space club is having some posters printed. The printer charges $250 plus $2 per poster. How can I figure out how many posters I could print for $1,000?

• 250x + 2 = 1000
• 1000 + 2x = 250
• 250 - 2x = 1000
• 2x + 250 = 1000

The length of a rectangular field is 75 yards. This is 3 yards more than twice the width. How could I find the width of the field?

• 2w + 3 = 75
• 2/w + 3 = 75
• 2w - 3 = 75
• 3w = 75

Grandpa Gump is 63 years old. His age is 2 years less than 5 times the age of Billy Gump. How could I find out how old Billy is?

• x/5 - 2 = 63
• 5x - 2 = 63
• 2x - 5 = 63
• 5x + 2 = 63

Three desks and a bookcase together weigh 157 pounds. The bookcase weighs 34 pounds. Which equation could help me find out how much each desk weighs?

• 34 + 3 = 157
• 34x + 3 = 157
• 3x + 34 = 157
• x + 34 = 157

Oshgosh would like to buy a new pair of skates for $115. So far he has saved $40. If he earns $7.50 per hour, how could I find out how many hours he must work in order to buy the skates?

• 40 + 7.50x = 115
• 40x + 7.50 = 115
• 40 + 7.50 = 115
• 40 + 17.50x = 115

Jo is thinking of a number. Four less than 9 times the number is 176. How could I find Jo's number?

• 9x - 4 = 176
• 4 - 9x = 176
• 9x + 4 = 176
• 4x + 9 = 176

Mo is thinking of a number. Eleven more than one third of a number is -1. Which equation would help me find Mo's number?

• 1/ 3 + 11 = -1
• 3x + 11 = -1
• 1/3x + 11 = -1
• 1/3x + 11 = 1