The scale is determined by dividing the respective measures of the two paintings.
Original painting mural scale factor
3 15 original / mural = 3 / 15 = 1 / 5
2 10 original / mural = 2 / 10 = 1 / 5
As you see the scale to comparing the original to the mural es 1 feet : 5 feet.
It means that the measures of the original painting must be multiplied by 5 to obtain the measures of the mural.
X=biscuit
x+15p (cake)
x+15p+2x=90p
3x+15p=90p
3x=90p-15p
3x=75p
x=75p÷3
x=25p
biscuit= 25p
cake= 25p+15p =40p
$7.38.........................................................
Sorry for the bad handwriting.