Yes you did the equation on this page correctly.
Set up a proportion.
x = length of actual wall
1/30=7/x
Cross multiply: x = 210 inches
Answer:
the total surface of the box is 486 cm 2
.......
Since the ratio of red drops to yellow drops is 55:88 and you have 1515 red drops, the yellow drops needed is given as
If 55=1515
88=?
88/55*1515=2424
therefore 2424 drops of yellow will be needed to make a shade of orange