Question 1082439
Let A be the number of 3/4 kg balls, B the 7/8 kg balls.
1.{{{A+B=28}}}
.
.
{{{(3/4)A+(7/8)B=22&1/2}}}
Multiply both sides by 8.
2.{{{6A+7B=180}}}
Multiply eq. 1 by 6 and subtract from eq. 2,
{{{6A+7B-6A-6B=180-28*6}}}
{{{B=12}}}
Now use either equation to solve for A.