Mr. abernathy bought a selection of wrenches for his shop and paid $78. he bought the same number of $1.50 and $2.50 wrenches, and half of that number of $4 wrenches. the number of $3 wrenches is one more than the number of $4 wrenches. how many of each did he buy?

X = # of $4 wrenches 
2x = # of $1.50 wrenches 
2x = # of $2.50 wrenches 
x+1 = # of $3 wrenches 

x(4.00) + 2x(1.50) + 2x(2.50) + (x+1)(3.00) = $78 
4x + 3x + 5x + 3x + 3 = 78 
15x + 3 = 78 
15x = 75 
x = 5 

Therefore, he has 5 $4 wrenches, 10 $1.50 wrenches, 10 $2.50 wrenches and 6 $3 wrenches. 
Verify: 
5($4) + 10($1.50) + 10($2.50) + 6($3) = $20 + $15 + $25 + $18 = $78

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