Take a good look at these equations and you should spot something interesting. The first and second equations have something in common, namely (10x+y). The first equation says that quantity equals 36, so substitute 36 for the expression in the second equation and what do we get? 36-(10y+x)=36. The two 36's cancel out leaving -(10y+x)=0. Forget about the minus in front of the brackets because minus zero and plus zero mean the same. We can just write simply 10y+x=0. The third equation says x-y=4. We have two simultaneous equations, both simultaneously true, in other words. We can write 10y+x as x+10y so that we can match the order of the variables in the third equation. Now subtract the third equation from this and we get x+10y-(x-y)=0-4. That's x+10y-x+y=-4. The x's disappear and we're left with 11y=-4, so y=-4/11. Substitute this value for y on any of the equations to find x. Let's pick x-y=4. So x=4+y and x=4+(-4/11) which is 4-4/11 or 40/11 (that is (44-4)/11). 40/11 is 3 and 7 elevenths. So the answer is x=40/11 and y=-4/11. Put these values in any of the equations to confirm the result.