1.The sum of all even numbers from 21 to 34 is :

22 + 24 + 26 + 28 + 30 + 32 + 34

= (22 + 24) + (26 + 34) + (28 + 32) + 30

= 46 + 60 + 60 + 30 = 196

2.The greatest distinct 2-digit number is 98

3.\(\dfrac{1}{5}\) of \(\dfrac{140}{4}\) is : \(\dfrac{140}{4}\times\dfrac{1}{5}=\dfrac{28}{4}=7\)

4. 2 dozens = 24

24 balls is 8 balls multiplied by : 24 : 8 = 3 (times)

The amount of money Lan has to pay to seller is : 48 : 3 = 16 (dollars)

5. 1 x 2 x 3 x 4 x 5 x ... x 11 = (2 x 5) x (1 x 3 x 4 x 6 x ...  x 11)

= 10 x (1 x 3 x 4 x 6 x ... x 11)

Hence,the last digit of the product is 0