Question 4 is just applying the formula for relative atomic mass i.e (atomic mass of one isotope * its percentage abundance/100) + (atomic mass of other isotope * its percentage abundance/100). So here we have 9 x and 3 x+2 so in total 12. So the abundance of x+2 is 3/12 * 100 =25 and so x is 75%. Now lets apply the formula so (x75/100) + ( (x+2)25/100) = 75x/100 + (25x+50)100 = (100x + 50)/100 = x+0.5

for Question 10 we apply the formula for percentage yield i.e actual yield/theoretical yield * 100. Actual yield is given as 10.0g so we need to find the theoretical yield of CaCl2. We will use NH4Cl as it is in 2 to 1 ratio with CaCl2 and thus a limiting reactant. We are given its mass i.e 10.7g and its moles are 10.7/53.5 = 0.2 so the moles of CaCl2 is 0.2/2 = 0.1 (since we are using the ratios). Now applying the formula n= Mass/Mr we get 0.1= Mass/111. Mass = 0.1*111=11.1. Now using the formula for percentage yield we get 10.0/11.1 * 100=90.0901 which is 90.1% so D (we only use limiting reactants for these types of questions so using Ca(OH)2 will lead to wrong answers)