You dont need to understand trig. You know the formula
Earth Convergence = Change in Long x Sin Mean Lat
to work out the .7660 from your example get your calculator press the sin button then enter 50 and press equals and you should get 0.766044443119. Put this figure back into your formula and you get your answer.
NOTE : If you type this in and get another number check that your calculator is in the deg mode and not rad or grad mode.
The second problem is solved in the same way but use the cos button.
If you can remember the formula and use the calculator you can get the answer every time.