Question

1. This sum is so hard that I cannot do it. (Make it simple sentence)
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Answer 1

Answer:

I can not do this hard sum

Explanation:

a simple sentence consists of a clause, subject and predict, so that's it.

Answer 2

The given complex sentence can be changed into a Simple Sentence as shown and explained below:

I cannot do such a hard sum.

or

I cannot do this hard sum.

Explanation

A simple sentence is made up of only one clause. It has just one subject and one predicate.  

Example  

Flowers are beautiful.