Hello,
let's start with cracking the two problems that already have an answer so we can understand how to solve 0,5s + 3t.
2s - 2 = 7
2s - 2 = 7 is an expression that says "the value of 2s minus 2 equals 7". That must mean that the original value of 2s was 9, because 7 plus 2 is 9.
That must mean that 1s equals 4.5, because 2s equals 9.
3s + 5t = 10
We then move on to the next problem, 3s + 5t = 10. We know that 1s equals 4.5, so 3s must equal 4.5 * 3 = 13.5. Wait a second, 13.5 is above 10! What is this!? Well, 5t must be a negative value, with a total value of (-3.5). We use brackets to make it easier to read negative values. It's easy then! 13.5 + (-3.5) = 10! Actually, not as easy as someone may think, negative numbers involve in a bunch of pesky rules, that's why the answer is 10 and not 17. I recommend you to read about said rules in case you already haven't.
So, 5t equals (-3.5). How much is 1t? Let's simply just take (-3.5) and divide it by 5, like this: (-3.5) / 5 = (-0.7). 1t equals (-0.7).
0.5s + 3t = ?
We now know that 1s equals 4.5 and that 1t equals (-0.7).
0.5s is half of the value of 1s. 0.5s = 4.5 / 2 = 2.25.
3t = (-0.7) * 3 = (-2.1).
A simple, readable, version of the problem would then be 2.25 + (-0.7).
2.25 + (-2.1) = 0.15
Finally, we have our answer, 0.5s + 3t = 0.15.
Hope this helped. Long answers scare me.
