51 x 52 = ( 50 + 1 ) x (50 +2)

(a + b) x (c +d) = a x (c + d) + b x (c + d) = a x c + a x d + b x c + b x d

It is this expansion you need to use. Think of it each variable in the first parent multiplied by each variable in the second and add them together

( 50 + 1 ) x (50 +2) = 50 x (50 + 2) + 1 x ( 50 + 2) = 50 x 50 + 50 x 2 + 1 x 50 + 1 x 2 = 50 x 50 + 3 x 50 + 2 = 2500 + 150 + 2 = 2652

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I would not ever get to 50 x 50 if tried to do it by hand or in my head 50 x 52 + 1 x 52 would be my start. 1 x 52 is 52 so we just add it at the end

Then try to get to a multiple of 10. 50 is 100 /2 so 50 x 52 = (100 * 52) /2 = 5200/2. You can directly get it is 2600 but you can split it too.

Ignore the 00 at the end. 52 / 2 = 26 or 50 / 2 + 2/2 = 25+1 =26. We cannot just add back the 00 at the end and get 2600

The result is no 2600 +51= 2651.

Tying to get to a multiple of 10 other another number that will be easy is a good idea 99 x 52 is directly hard but 100 x 52 - 1 x 52 is not. You get 5200 - 52 and you can get 5148 from that