Ok this one is quite a difficult one. You could use trial and error.
Here are some equations to look at: see if you can understand these
sp + hr = 18
hr + m = 21
sp + m = 23
This is from the information you have been given. You could list all the numbers that equal 18.
eg. 1 + 17; 2 + 16; 3 + 15 etc and then do the same for the others and see if this help you solve the problem. Always remember to check if your answer is correct.

Ok if you want to try using equations here is help with the first step.
how are you going to solve sp + hr = 18?
sp = 18 - hr.
How are you going to solve the second equation which is hr + m = 21.
That is all for tonight guys. Will give you more clues tomorrow. See you tomorrow. Bye