let

x = the weight of Kevin

y = the weight of Ken

z = the weight of Kirk

Together Kevin and Ken weigh 170 pounds:

x + y = 170

Kevin and kirk weigh a total of 155 pounds:

x + z = 155

Kirk and ken weigh a total of 175 pounds:

y + z = 175

by solving the system of equations:

x + y = 170

x + z = 155

y + z = 175

we find:

x = 75 lbs

y = 95 lbs

z = 80 lbs

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Kevin weighs 75 lbs.

Ken weighs 95 lbs.

Kirk weighs 80 lbs.