Let R and M be the numbers of serving of roast beef and mashed potato.
25R + 2M = 29 =====> 1
11R + 25M = 61 =====> 2
From (1): 2R = 29 - 25M
or M = 29/2 - 25/2 R
Substitute this into (2):
11R + 25(29/2 - 25/2 M) = 61
11R + 362.5 - 312.5R = 61
301.5 = 301.5R
R = 1
Using R = 1, we can get
11(1) + 25M = 61
25M = 50
M = 2
Therefore, you will need ONE SERVING of Roast Beef and TWO SERVINGS of Mash Potatoes.
Hope that helps!!!! : )