Question 1081555
Let A be the number of trips for 4.5 ton truck, B the number for 11 ton truck.
1.{{{A+B=28}}}
and
2.{{{4.5A+11B=204}}}
 Multiply eq. 1 by 11 and subtract from eq. 2,
{{{4.5A+11B-11A-11B=204-308}}}
{{{-6.5A=-104}}}
Solve for A, then use either equation to solve for B.