SOLUTION: In the design of a bridge, an engineer determines that four fewer 18-m girders are needed for the span length than 15-m girders. How many 18-m girders are needed?

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Question 144863This question is from textbook Basic Mathematics with Calculus
: In the design of a bridge, an engineer determines that four fewer 18-m girders are needed for the span length than 15-m girders. How many 18-m girders are needed? This question is from textbook Basic Mathematics with Calculus

Answer by Alan3354(69443) About Me  (Show Source):
You can put this solution on YOUR website!
L, the length is
x times 15, or
x-4 times 18
So, 15x = 18(x-4)
15x = 18x-72
-3x = -72
x = 24
15 of the 24' girders = 360 feet.
18 of the 20' girders = 360 feet.