In a box there are n red marbles and three times as many black marbles. There are 36 marbles in the box. How many red marbles are in the box?

it is 12

Step-by-step explanation:

for n= 36 divided by 3

distance covered by pamela in 9 l of fuel = 99km

let us apply unitary method here.

distance covered by pamela in 1 l of fuel =99/9 =11 km

now let us find distance covered by pamela if she has 12 l of fuel.

distance covered in 12 l = distance covered in 1 l of fuel * 12

we know that distance covered by here in 1 l is 11 km.

so distance covered in 12 l of fuel = 11*12 = 132 km.