Small tub size S, large tub size L

2S + 2L = 2.5

4S + 3L = 4.3

Rewrite the first equation as

2L = 2.5 - 2S

L = 1.25 - S

Plug L into the 2nd equation

4S + 3 (1.25 -S) = 4.3

4S + 3.75 - 3S = 4.3

S = 4.3 - 3.75 = 0.55

Stick S into the first equation

2 * 0.55 + 2L = 2.5

2L = 2.5 - 1.1 = 1.4

L = 0.7