The grasshopper, the frog and the mouse had a jumping contest. The grasshopper jumped 39 inches. The grasshopper jumped 19 inches farther than the frog and the mouse jumped 12 inches lesser than the frog. How far did the mouse jump?
The frog jumped x inches.
The mouse jumped y inches.

The grasshopper jumped 39 inches.
The grasshopper jumped 19 inches farther than the frog.

39 = x + 19

The mouse jumped 12 inches lesser than the frog.

y = x - 12

Now we have two equations with two unknowns. We can solve this system of linear equations.

x + 19 = 39
x - 12 = y

x = 28
y = 16