Question 760518
Let the baby sister's weight be {{{x}}} kg
Then, Krista's weight = {{{8*x}}} as given in the problem.

Combined weight = {{{ x + 8*x = 54}}}

or 

{{{9*x = 54}}}

Dividing both sides by 9,

{{{x = 54/9 = 6}}}

So, baby sister weighs 6 kg

Krista's weight = {{{8*6 = 48}}} kg

Got it?