let x be the value of the horse in dollars

12 months wage is 240 + x => one month wage is (240 + x)/12

7 month wage is 100 + x => one month wage is (100 + x)/7

so we have:

(240 + x)/12 = (100 + x)/7

=> (240 + x)/12 - (100 + x)/7 = 0

=> (1680 + 7x - 1200 - 12x)/84 = 0

=> 1680 + 7x - 1200 - 12x = 0

=> 480 = 5x

=> x = 96

so the value of the horse is $96