Math
posted by Gigi
Q. When your doing a multiplication problem, for example 84 times 37 = 3,108, how do you check the answer without a calculator?

A. I'm kinda confused on how to check the answer, do i do 84 times 400??

Reiny
here is a very old trick:
add up the digits of each factor
87> 8+4 = 12 , do it again  1+2 = 3
37 > 3+7 = 10 > 1+0=1
multiply them : 3*1 = 3
take the answer and add up the digits
3108 > 3+1+0+8 = 12  1+2 = 3
same result!, extremely high probability that our answer is correct
This is called "casting out nines"
Respond to this Question
Similar Questions

Math,help
how do i evalute this problem if possible. i will decribe it it has an index of 3 then a squaroot sign then for the radicand (64) since the cube root of 64 is 4(4 times 4 times 4 = 64).. the () part, you just tack on, since a () … 
math,correction
are these correct now i hope? Problem #1 simplify (m^2)/(8) divided by (m^5)/(16) My answer: (2)/(m^4) Problem #2 multiply (3x30)/(x^24x) times (8x)/(10x) My answer: (6)/(x) No. 2/m^3 (3x30)/(x^24x)* (8x)/(10x) 3*(x10)*8x /(x10)(1)(x)(x4) 
Mathwhich is correct
1 euro is worth 1.2 U.S dollars if I budget 150$ per night for a hotel room. What is the maximum number of euros I should spend for a room? 
GCF Math Check
Can you see if my answers are correct? Directions find the GCF: (I did the factor tree for each) 40; 2^3 times 5 128; 2^7 GCF: 2 90; 3^2 times 2 times 5 105; 5 times 3 times 7 180; 3^2 times 2^2 times 5 GCF: 5 3x^3; what is that suppose 
Algebra
I have this math probelm z(3z)/(2z)=(6)/(z2) in my book it says to solve the equation. I have worked this problem many times and I keep comming up with different answers. When I use an online calculator to check my work it is giving … 
Math check
Factor each expression: x^2144/x+4 times x^216/x+12 This is what I got: (x+12)(x12)/x+4 times (x8)(x+2)/x+12 How is the answer x^216x+48 
Math 8R: Homework Check  Part 4
Part 4: Translate and solve: 25. 6 less than twice a number is ten Answer : 2n  6 = 10 (answer: n = 8) 26. The quotient of a number and 4 plus 3 is 8 Answer: n/4 + 3 = 8 (answer: n = 29) 27. 4 less than three times a number is 28 … 
Math
Write the algebraic expressions for the following. Please check and correct me if I'm wrong!! 1. 3 less than 5 times a number. Answer: 5x3 2. 18 fewer than half a number. Answer: 1/2x18 3. 3 more than the quotient of a number and … 
6th Grade Math
My Question: When Im doing multiplication, do i check it by using 2? 
Math
Please check to see if I did this correctly. This is a 2 part math problem. 1. Janelle has d dimes and n nickles. Her sister has 5 times as many dimes and 6 times as many nickles as Janelle has. Write the sum of the number of coins …