Help please? And if you could show how you solved it

Asystem of equations has 1 solution. if 4x-y=5 is one of the equations , which could be the other equation ?

Awater holding tank measures 105 m long, 50 m wide, and 8 m deep. traces of mercury have been found in the tank, with a concen- tration of 60 mg/l. what is the total mass of mercury in the tank? answer in units of kg.

Prove that 16^4-2^13-4^5 is divisible by 11.