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Find out the correct number to replace the question mark.

Source: Beijing Aidi International School summer test (2018-08-21). Author: 坦途国际教育

You are correct. The answer is 9.

Explanation

The first line has 3 bears with hats totaling 21. So a bear with a hat is 21/3 = 7.

Then

The next line is 2 sets of a bear in a bus plus a bear in a hat totaling 19. So we know a bear in a bus is (19-7)/2 = 6.

Then

Next is a hat, a bear in a bus, and a bear in a hat totaling 15. So we know a hat is 15-6-7=2.

Then

Now that we know a hat is 2, a bear must be 5 (7-2). If a bear is 5, than a bus is 1 (6-5)

Thus

A hat is 2, a bear is 5, and a bus is 1. That makes the last line 5 + 4*1 which does equal 9.

Adding neater formatting but here is solution:

The key to this puzzle is attention to detail about what each image truly is

For the first line:

We have 3 bears and 3 hats. This adds up to 21 meaning a bear + a hat = 7.

Next we have to look at the third line:

In this line we have a bear with a hat (7), a hat and a bus with a bear. In total then we have a bus of unknown value and 2 bears with hats valued at 7 each. To get the total sum of 15 an empty bus must equal 1 (15-(2x7)).

Back to the second line now:

There are 2 buses with a hat-less bear and one bear with a hat (7). Since each bus is worth 1 and the bear with hat is worth 7, subtracting 9 from 19 leaves 10 for the value of two bears and 5 for the value of each individual bear.

Value of a hat:

Since a hat and a bear is 7 and a bear is 5, a hat is worth 2.

Final Line:

To recap: A bear without hat is 5, two hats will be 4 and a bus is 1. 1x4 (BEDMAS/PEDMAS) is still 4. All that is left is 4 + 5 which equals 9.

Let t be the number of teddy bears, h hats and b buses. Then the equations are:

$3t+3h=21 tag1$

$3t+h+2b=19 tag2$

$2t+2h+b=15 tag3$

and we want to know:

$t+(2htimes b)=t+2hb$

Gaussian elimination tells us the answer is:

$t=5, h=2, b=1$, therefore $t+2hb=9$.

but there is a quick way:

From (1) $t+h=7$. From (3) $b=1$. From (2) and (3), $t-h=3 implies t=5, h=2$.

Pig (with a hat) = 7

Bus (with a pig) = 6

Hat = 2

Pig = 5

Bus = 1

Hence pig (without hat) + (2 * hat) * bus (without a pig) = 5 + 4 * 1 = 9