To find the percent mass of solute in a solution you have to divide the mass of solute by the mass of solution and multiply by 100%.
First find the mass of solution by adding the mass of solute to the mass of the solvent. 4.40g +10.3g = 14.7g
4.40g* 100% divided...

To solve this you can set up a proportion. If the original number is 100% 54 would be 108% of the original. (Since it was increased by 8%).
54-----108%
x-------100%
solving the proportion gives you
54 * 100= 108 * x
x=...

First you have to solve for one of the variables in terms of the other. Take one of the equations for example a+b=1000 and solve for a to get a=1000-b. Now you can substitute this expression into the other equation instead of a. 7a+15b=11504 becomes
7(1000-b)+15b=11504...