Question

In a bag containing only blue, red and green marbles, all but 15 are blue, all but 13 are red and all but 12 are green. how many are red?

Answer 1

7 marbles are red ! 

THIS IS HOW :
total marbles =x 
no. of blue marbles=x-15
no. of red marbles =x-13
no. of green marbles = x-12

so , x-15 +(x-13)+x-12 = x 
      3x-40 =x
      x =20


RED MARBLES = x-13 = 20-13 = 7

Answer 2

Concept:

Replace the unknown quantity by a variable and then make a suitable mathematical equation using condition by which we get the required result.

Given:

Given that, In a bag containing only blue, red and green marbles, all but 15 are blue, all but 13 are red and all but 12 are green.

Find:

The number of red marbles in the bag.

Solution:

Let the number of total marbles in bag be x.

All but 15 marbles are blue

So number of blue marbles = x-15

All but 13 marbles are red

So number of red marbles = x-13

and all but 12 marbles are green.

So number of green marbles = x-12

According to condition,

(x-15) + (x-13) + (x-12) = x

(x+x+x) - (15+13+12) = x

3x - 40 = x

3x-x =40

2x = 40

x = 40/2

x = 20

Thus the number of total marbles = 20

Then the number of red marbles = x-13 = 20-13 = 7

Hence the number of red marbles in the bag is given by 7.

#SPJ2