lets write the equations with shortcodes A for apple, L for lemon, W for watermelon and T for tomato

3A = 30 => A = 10

2L + A = 40 => 2L = 40 -A = 30 => L = 15

from 4th T + T^2 = 12 => T => T(1+T) = 12 => T = 3

from 3rd T + 2W => 24 => 2W = 24 – T = 21 => W = 21/2

the last one

W + L*T => 21/2 + 15*3 => 10.5 + 45 => 55.5