Question 254628
The perimeter is 75 can be expressed as
{{{P = a + b + c}}} = {{{75}}}
Now 
b/a = 3/5 means that 
(i) b = (3/5)a
and
c/a = 7/5 means that 
(ii) c = (7/5)a
by substituting (i) and (ii) into perimeter we get
{{{a + (3/5)a + (7/5)a = 75}}}
and then
multiply by 5 to get
{{{5a + 3a + 7a = 375}}}
{{{15a = 375}}}
{{{a = 25}}}
{{{b = 15}}}
{{{c = 35}}}