Cats generally have gray fur which is caused by a dominant gene. the recessive condition results in orange fur. two heterozygous cats are crossed. what proportion of the offspring will have orange fur if this type of inheritance follows simple mendelian inheritance?

The answer is 25%.

This is an example of Mendelian monohybrid cross.
If:
A - (dominant) allele for gray fur
a - (recessive) allele for orange fur

Then:
AA - dominant homozygous
Aa - heterozygous
aa - recessive homozygous

The cross between two heterozygous will be:

Parents:            Aa        x        Aa
Offspring:     AA      Aa      Aa       aa

Thus, the offspring will be: 1 out of 4 (25%) dominant homozygous (AA), 2 out of 4 (50%) heterozygous (Aa), and 1 out of 4 (25%) recessive homozygous (aa).
Since the recessive condition results in orange fur, only recessive homozygous (aa) will have orange fur, which is 25% of all the offspring.

If the dominant gene in cats is the one which gives them gray fur and the recessive condition is responsible for orange fur, then two heterozygous cats which would be crossed would have offspring: 

Ff (cat 1) Ff (cat 2)

FF Ff Ff ff

75% of the offspring would have gray fur (big F) and only 25% of the offspring would have orange fur (two small ffs)