Substitution is the process of replacing an unknown variable with a number that is known and evaluating the answer to the problem.
eg If a = 5 and b = 7, then a + b = 5 + 7 = 12
eg If a = 10 and b = 3, then 2b + a = 2 ✕ 3 + 10 = 16
eg If diameter, d = 10cm, then the circumference of a circle = πd = π ✕ 10 = 31.4cm (1dp)