Simplify the right side first: 125000*4/5*t=100000t. To get rid of the decimal we can multiply both sides by 10: 75=1000000t. So t=75/1000000=0.000075 or 7.5*10^-5.
That may not be the answer you're looking for. Assuming the cost of the bulldozer was 125,000, after the first year it would depreciate by 1/5=25,000. At the start of the second year it would be worth 100,000. At the end of the second year it would depreciate by another fifth=20,000, so it would be worth 80,000 at the start of the third year. By the end of the third year it would depreciate by another 16,000 and be worth 64,000 at the start of the fourth year. At the start of the fifth year another 12,800 would be lost making its value 51,200. At the sixth year it would be worth 40,960 and at the seventh year 32,768. After another sixth months, assuming a prorata depreciation it would lose only one tenth its value 3,276.80 so would be worth 29,491.20.