C assignment

ANybody tell me what the problem is here? Im sure its something rediculously simple I just cant see it.

Bruce

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/* File: decay.c
Programmer: Bruce Reid Date: October 11, 2004
COurse: ENGM2081

The time decay of a radioactive isotope is given in terms of its
half-life, H, the time lapse required for the isotope to decay to
one-half of its original mass. The isotope strontium 90 has
a half-life of H= 28 years.

This program computes and prints in table form (to screen and to the
file decay.out) the amount oh this isotope that remains after each year
up to n years, given initial amoun tin grams.

The values of n and initial amount are read interactively. The amount
of Sr90 remaining after y years can be computed from the formula:

amount = (initial_amount)2^(-y/H)

*/

#include <stdio.h>

int main(void)

{
FILE* outfile;

outfile = fopen("ftest.out", a) ;

float initialamount, amount, y, n, exponent ;
float H=28.0;
printf("How many grams of Strontium did you begin with?") ;
scanf("%f", &initialamount) ;
printf("\nHow many years has the sample been decaying?") ;
scanf("%f", &y) ;

for(n=0; n<=y ; n++)
{
exponent = pow(2,(-n/H);
amount= (initialamount)*exponent ;
printf("Year: %f\tAmount Remaining: %f",n,amount);
fprintf(outfile, "Year: %f\tAmount Remaining: %f",n,amount);
}

return 0;

}

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