# 100+ Challenging math riddles to keep your mind sharp

#### John,Mark and Henry Age -Math riddle

###### 61. Math Riddles

John, Mark, and Henry are three brothers. Interestingly, their current age is prime. What's more interesting is that difference between their ages is also prime. How old are they?

Mark is 2, John is 5,
and Henry is 7.

Explanation :

Age diff,
7 - 2 = '5' is prime;
7 - 5 = '2' is prime;
5 - 2 = '3' is prime.

#### The Wedding Shower Gift - Math Riddle

###### 62. Math Riddles
`Janie's friends were chipping in to buy her a wedding shower present. At first, 10 friends chipped in, but 2 of them dropped out. Each of the 8 had to chip in another dollar to bring the amount back up. How much money did they plan to collect?`

`Explanation :  \$40 (10 at \$4, or 8 at \$5)`

#### John was six years old - Math Riddle

###### 63. Math Riddles
`When John was six years old he hammered a nail into his favorite tree to mark his height. Ten years later at age sixteen, John returned to see how much higher the nail was. If the tree grew by five centimeters each year, how much higher would the nail be?`
`Explanation :  The nail would be at the same height since trees grow at their tops.`

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#### How many circles - Math Riddle

###### 64. Math Riddles

How many circles contain the black dots?

Explanation :

If you look carefully at the image, you'll see that the dots are placed in intersection of many circles. So, The correct answer is 12.

#### Duck Rabit Dog Weight - Math Riddle

###### 65. Math Riddles

Given that the weight of a Rabbit and a Duck is 10 kilograms, the weight of a Dog and a Rabbit is 20 kilograms, and the weight of a Dog and a Duck is 24 kilograms, what is the total weight of a Dog, a Rabbit, and a Duck?

Explanation :

Let the duck be ‘Du’

The dog be ‘Do’

The rabbit be ‘R’.

From the above picture,

we can form equations as,

R + Du = 10kg.…equation (1)

R + Do = 20kg….equation (2)

Do + Du = 24kg…equation (3)

& R + Do + Du = ?

Rearranging the equations,

equation (1) = R + Du = 10kg

Du = 10kg – R

equation (2) = R + Do = 20kg

Do = 20kg – R

Putting the values of Do & Du in equation (3),

Do + Du = 24kg

20kg – R + 10kg – R = 24kg

30kg – 2R = 24kg

2R = 6

Rabbit, R = 3kg

Substituting the value of R, in equation (1) & equation (2)

We get,

equation (1), R + Du = 10kg

3 + Du = 10kg

Duck, Du = 7kg

Also, equation(2), R + Do = 20kg

3 + Do = 20kg

Dog, Do = 17kg.

Adding the values of,

Do + Du + R = 3 + 7 + 17

Dog + Duck + Rabbit = 27kg.

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#### Move just 2 matchsticks - Math Riddle

###### 67. Math Riddles

Move just 2 matchsticks to make the equation Correct.

Explanation :

9-9=0. Move one from the addition sign to make it a subtraction sign, and put it to 3 to make 9, 2. move one from 8 to make it 9.

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#### 10 small boxes into a carton- Math Riddle

###### 71. Math Riddles

A merchant can place 8 large boxes or 10 small boxes into a carton for shipping. In one shipment, he sent a total of 96 boxes. If there are more large boxes than small boxes, how many cartons did he ship?

Explanation :

7 large boxes (7 x 8 = 56 boxes)
4 small boxes (4 x 10 = 40 boxes)
So the answer is 11 total cartons for 96 boxes

#### A 35 year old woman - Math Riddle

###### 72. Math Riddles

A 35 year old woman has 3 children. If you multiply the 3 children's ages, you get 36. If you add up the children's ages it's 13. And her oldest child plays basketball for 3 years. How old are the kids?

They are 2, 2, and 9.

Explanation :

If you list out the trio of factors that multiply to 36, you get: 1 1 36 = 38 1 2 18 = 21 1 3 12 = 16 1 4 9 = 14 6 6 1 = 13 2 2 9 = 13 2 3 6 = 11 3 3 4 = 10 If the sum is 13, as you can see, there are 2 options. When she sa ys her 'oldest' you know it cannot be {6, 6, 1} since she would have two 'older' sons, not an 'oldest'.

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#### Find the value - Math Riddle

###### 73. Math Riddles

Can you find the value of the missing number?

Explanation :

The outer number is the sum of the numbers in the two segments immediately across the line connecting big Circle to small Circle.

i.e.
5+6=11,
6+2=8,
2+8=10,
8+1=9,
1+4=5,
4+7=11,
7+9=16
therefore,
9+5 =14

#### An Athlete jumping - Math Riddle

###### 75. Math Riddles

An Athlete is jumping. However, every time she jumps she gets a bit more tired, and every jump goes 1/2 as far as her prior jump. Now, for her very first jump, she goes 1/2 of a foot. On her second jump, she goes 1/4 of a foot, and so on and so forth. How many jumps does it take for her to travel 1 foot?

She will never get to the 1-foot mark because you keep adding smaller and smaller amounts!

Explanation :

We will find the maximum value of this process, when this is carried out infinite times.

This process goes like a Infinite GP, whose
First term,a = 1/2ft.
Common Ratio, r = 1/2ft.

Maximum value of this process is One Foot when the athlete jumps forever or Practically, she will never complete her 1 foot.

Hence, Athlete will never complete. her complete .
Here,
As shown in Number line, since we are taking half distance of the previous one.
Starting from 1/2 ft. ,and ending at 1 foot.
The value always tends towardd 1 but never reaches one.

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