Let A be the area, L be the length and W be the width
"A rectangular garden is 4ft longer than it is wide" tells us L = W + 4
Also area = length x width, so A = LW,
"If the width is decreased by 1 ft., and the length is increased by 5ft., the area is increased by 15 square ft" tells us...
A swing from left to right is only half a complete cycle ... the complete cycle is a swing from left to right, and back left again. This means the pendulum has a period of 2 seconds, not 1. That's the trick.
The period and the length of a pendulum are related by the equation
T = 2π√(L/g) and...