I would like to share some Math Tricks with you to show how interesting and easy it can be done mentally. E.g.:

1. 
55 multiply 55 = 3025
When we have two numbers same and ending with 5, last two digits of the answer will always be 25 as a rule and first digits i.e. 5 and 6 multiply to get 30 and answer =3025. 
2. 
45 multiply 55= 2475
When we have two numbers with a differentce of 10 and both ending with 5, last two digits of the answer will always be 75 as a rule .d first digits 4 and 5 multiply to get 20 and add the smaller number 4 to 20 to get 24 and the answer = 2475. 
3. 
42 multiply 11=462
78 multiply 11=858
When we multiply 42 and 11, Just write 4 and 2 leaving sore space and add 4 and 2 to get 6 and the answer 462. And for 78 times 11 write 8 for the last digit of the answer and adding 7 and 8 we get 15 so wrife;5 and carry over 1 and add to 7 to get 8 and the answer 858. 