Binomial expansion of (1+4x)^5 up to x^2

This question can be treated like a normal binomial expansion question which is commonly seen at A level. The standard binomial expansion is (1+x)ⁿ = 1 + nx + (n(n-1)(x²))/2! where 2! is 2 factorialSo in this question let u=4x, n=5(1+u)^5 = 1 + 5u + (5)(4)(u²)/(2)(1)(1+u)⁵= 1 + 5u + 10u²now we need to substitute in u=4xso (1+4x)⁵ = 1 (5)(4)x + 10(4x)²(1+4x)⁵=1+20x+160x²