Answer this equation

The solution to the box is

a = 2.1

b = 5.9

c = 0.9

d = 10

Step-by-step explanation:

To answer the equation, we simply name the boxes a,b,c and d.

Such that

a + b = 8 ---- (1)

b - c = 5 ------ (2)

d * c = 9 ------ (3)

a * d = 21 ------- (4)

Make d the subject of formula in (3)

d * c = 9 ---- Divide both sides by c

d * c/c = 9/c

d = 9/c

Substitute 9/c for d in (4)

a * d = 21

a * 9/c = 21

Multiply both sides by c

a * 9/c * c = 21 * c

a * 9 = 21 * c

9a = 21c ------ (5)

Make b the subject of formula in (1)

a + b = 8

b = 8 - a

Substitute 8 - a for b in (2)

b - c = 5

8 - a - c = 5

Collect like terms

-a - c = 5 - 8

-a - c = -3

Multiply both sides by -1

-1(-a - c) = -1 * -3

a + c = 3

Make a the subject of formula

a = 3 - c

Substitute 3 - c for a in (5)

9a = 21c becomes

9(3 - c) = 21c

Open bracket

27 - 9c = 21c

Collect like terms

27 = 21c + 9c

27 = 30c

Divide both sides by 30

27/30 = 30c/30

27/30 = c

0.9 = c

c = 0.9

Recall that a = 3 - c

So, a = 3 - 0.9

a = 2.1

From (1)

a + b = 8

2.1 + b = 8

b = 8 - 2.1

b = 5.9

From (3)

d * c = 9

Substitute 0.9 for c

d * 0.9 = 9

Divide both sides by 0.9

d * 0.9/0.9 = 9/0.9

d = 9/0.9

d = 10.

Hence, the solution to the box is

a = 2.1

b = 5.9

c = 0.9

d = 10

answer: it is d

step-by-step explanation: