How do i work this sum out without a calculator?

It's called long multiplication

The correct answer has already been given. I wonder what education system has the responsibility for producing students who are incapable of carrying out simple multiplication. I recently saw a TV programme in which an employer said that many of the applicants for jobs are neither numerate nor literate. When I see some of the questions asked on Y/A I can understand it. This question should be simple to a student in primary school.

i need 2 work out 77*79 without useing a calculator --------------------------------------… 77 x 79 = (80-3) x (80-1) then multiply FOIL, then sum. 80 x 80 +(-3 x -1) -80x3 -80x1 6400 +3 -240 -80 = 6400 - 320 + 3 = 6083

I lay it out with 77 at the top and 79 underneat, then you times 7*9 = 63 so 3 goes in the units column and 6 in the tens (a small 6 under where the next number will go) then do the same again 7*9 = 63, so then add the 3 to the 6 in the tens column making 9 and add the 6 in the hundreds column, so you should have 693 then on the next line down put a 0 in the units column and do 7*7 = 49, put the 9 in the tens column and a small 4 in the hundreds column, then 7*7 again =49, so add the 9 to the 4 making 13, put the 3 in the hundreds column and the 1 (small again) in the thousands column, next add the 4 and 1 making 5 in the thousands column, so you should have 5390, then add that to the top number, so that's 693+5390 = 6083 which is your answer. Sorry it's a little complicated, but it's something I find easier to show than tell.

(n-3) * (n-1) where n=80 n^2-4n + 3 6400 - 320 +3 =6083 of course if you knew what 78^2 (78 squared ) was it's even easier (77 * 79) = (n+1) (n-1) where n =78 (78-1) * (78 +1) = 78^2 -1

You can split it down into: (77 x 70) + (77 x 9) or to make it simpler: (77 x 7 x 10) + (77 x 9)

(70 + 7)*(70 + 9) = 70*70 + 7*70 + 9*70 + 7*9. (You should have no problems from here.)

77 × 7 = 539 77 × 9 = 693 539 + ..693 ---------- 6083 That's how I calculate in my mind.

77* 79 ----------- =693 + 539 ======= 6083