P/L=4/7 and P=L-12 so
(L-12)/L=4/7 multiply both sides by 7
(7L-84)/L=4 multiply both sides by L
7L-84=4L subtract 4L from both sides
3L-84=0 add 84 to both sides
3L=84 divide both sides by 3
L=28, and since P=L-12
P=16
So they had 28+16 = £44 in total.