Question 33136
First try to write the given informations in the form of equations.
1. let K be keith's weight ,H be henry's weight , B be beth's weight.


then u can write the given infy's as,

 eq 1.   K = 20 + B ,
 eq 2.   H = 2 * B -30,

K & H ARE SAME  K = H,

Therefore  eq 1. and eq 2. are same so they can be equated,

 20 + B = 2* B - 30

 20 + 30  = 2* B - B  ( RE-ARRANGE NUMBERS ON ONE SIDE AND VARIABLE "B" ON OTHER SIDE ,u've to change the sign when doing so)

Therefore B = 50, The weight of BETH is 50kg..

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