20. Add: $a^4-2a^{3}b+3a{b}^3+4a^2{b}^2+3b^4 \mathrm{and} -2a^4-5a{b}^3+7a^{3}b-6a^2{b}^2+b^4$

21. A rectangular grassy lawn measuring 40 m × 25 m is to be surrounded externally by a 2 m wide path. Calculate the cost of levelling the path at the rate of Rs 8.25 per square m.

22. The angles of a triangle are $\left(x-40\right)°, \left(x-20\right)° \mathrm{and} \left(\frac{1}{2} x-10\right)°$. Find the value of x.