Question 471378
The equation is 
{{{(3p)/5 + (4(1-p))/10 = 7/15}}}

<==> {{{(3p)/5 + (2(1-p))/5 = 7/15}}}

==> 3p + 2(1-p) = 7/3, after multiplication by 5.

==> 3p + 2 - 2p = 7/3

==> p = 7/3-2

==> p = 1/3.