A cereal sample was analysed for Calcium content as follows:
The absorbance of this solution was found to be 0.351.
A 7ppm calcium solution gave an absorbance of 0.254

You have slightly (~1.5 times) higher absorbance (.35) than a solution of 7 ppm calcium (0.24)


  step 1: 7 ppm/ (0.351 x 0.254) = 78.515 ppm ca in final dilute sample.   


This is not correct. You should have proportion
7/0.254 = x/0.351   leading to x = 7*0.351/0.254

You get 10 times higher value. This is the first error. You should have done 7*(0.351/0.254) = 9.67 ppm.

In 50 mL it is A, but when it was in 5 mL it was 50/5 = 10 times higher. 
7*(0.351/0.254)*(50/5)=96.7 ppm. 

This means you have 96.7 mg calcium per 1 kG of cereal (ppm is one millionth, and thus, mg per kG). Then one tenth of that amount of cereal (0.1 kg/1 kg) contains 1/10 of the calcium. that is 9.67 mg.

Once again, you should have had these steps: 
So, you should have these steps: 

(1) Since the final sample absorbs .35/.254 ~1.38 times more it is 1.38 times more concentrated than 7 ppm. That is 9.67 ppm. 

(2) It was diluted 50/5= 10 times, so it was 10 times more concentrated before. That is 96.7 ppm. 

(3) For 100 g of cereal: 100 000 mg * 96.7 /1 000 000 = 96.7 mg /10 = 9.67 mg.