First we have to know that how we can double a number and half a number.
Halving the number means dividing the number by 2 and doubling the number means multiply by 2.
Now the given expression is "28 x 50".
half of the number 28 = 28/2 = 14
and double of the number 50 = 50 x 2 = 100
Now, multiply 14 and 100.
14 x 100 = 1400 answer.
There is one other method also to solve this;
Double of 50 is 100,
so 28 x 100 = 2800
now half of the answer is 2800/2
= 1400